Fix a couple C++ build issues

Fix a couple C++ build issues

Building mingw GDB with --enable-build-with-cxx shows:

../../binutils-gdb/gdb/python/py-unwind.c:500:45: error: cannot convert 'cached_frame_info::reg_info*' to 'pyuw_prev_register(frame_info*, void**, int)::reg_info*' in initialization
   struct reg_info *reg_info = cached_frame->reg;
                                             ^
../../binutils-gdb/gdb/python/py-unwind.c:501:60: error: invalid use of incomplete type 'struct pyuw_prev_register(frame_info*, void**, int)::reg_info'
   struct reg_info *reg_info_end = reg_info + cached_frame->reg_count;
                                                            ^
../../binutils-gdb/gdb/python/py-unwind.c:500:10: error: forward declaration of 'struct pyuw_prev_register(frame_info*, void**, int)::reg_info'
   struct reg_info *reg_info = cached_frame->reg;
          ^
../../binutils-gdb/gdb/python/py-unwind.c:505:37: error: cannot increment a pointer to incomplete type 'pyuw_prev_register(frame_info*, void**, int)::reg_info'
   for (; reg_info < reg_info_end; ++reg_info)
                                     ^
../../binutils-gdb/gdb/python/py-unwind.c:507:29: error: invalid use of incomplete type 'struct pyuw_prev_register(frame_info*, void**, int)::reg_info'
       if (regnum == reg_info->number)
                             ^
../../binutils-gdb/gdb/python/py-unwind.c:500:10: error: forward declaration of 'struct pyuw_prev_register(frame_info*, void**, int)::reg_info'
   struct reg_info *reg_info = cached_frame->reg;
          ^
../../binutils-gdb/gdb/python/py-unwind.c:508:68: error: invalid use of incomplete type 'struct pyuw_prev_register(frame_info*, void**, int)::reg_info'
         return frame_unwind_got_bytes (this_frame, regnum, reg_info->data);
                                                                    ^
../../binutils-gdb/gdb/python/py-unwind.c:500:10: error: forward declaration of 'struct pyuw_prev_register(frame_info*, void**, int)::reg_info'
   struct reg_info *reg_info = cached_frame->reg;
          ^
../../binutils-gdb/gdb/python/py-unwind.c: In function 'int pyuw_sniffer(const frame_unwind*, frame_info*, void**)':
../../binutils-gdb/gdb/python/py-unwind.c:574:70: warning: invalid conversion from 'void*' to 'cached_frame_info*' [-fpermissive]
                             reg_count * sizeof (cached_frame->reg[0]));
                                                                      ^
../../binutils-gdb/gdb/python/py-unwind.c: In function 'void pyuw_on_new_gdbarch(gdbarch*)':
../../binutils-gdb/gdb/python/py-unwind.c:636:47: warning: invalid conversion from 'void*' to 'pyuw_gdbarch_data_type*' [-fpermissive]
       gdbarch_data (newarch, pyuw_gdbarch_data);
                                               ^
../../binutils-gdb/gdb/python/py-unwind.c:647:29: warning: invalid conversion from 'void*' to 'const frame_data*' [-fpermissive]
       unwinder->unwind_data = (void *) newarch;
                             ^
../../binutils-gdb/gdb/python/py-unwind.c: At global scope:
../../binutils-gdb/gdb/python/py-unwind.c:699:21: error: redefinition of 'PyTypeObject pending_frame_object_type'
 static PyTypeObject pending_frame_object_type =
                     ^
../../binutils-gdb/gdb/python/py-unwind.c:96:21: error: 'PyTypeObject pending_frame_object_type' previously declared here
 static PyTypeObject pending_frame_object_type
                     ^
../../binutils-gdb/gdb/python/py-unwind.c:749:21: error: redefinition of 'PyTypeObject unwind_info_object_type'
 static PyTypeObject unwind_info_object_type =
                     ^
../../binutils-gdb/gdb/python/py-unwind.c:99:21: error: 'PyTypeObject unwind_info_object_type' previously declared here
 static PyTypeObject unwind_info_object_type
                     ^

The first kind of error is caused by the embedded struct definition,
so move it out of the parent struct.

The second kind of error is caused by forward declaring a static
global variable, which works in C, but not in C++ (or C with
-fno-common).  Make it using extern instead, like done in other
similar cases.

gdb/ChangeLog:
2015-05-15  Yuanhui Zhang  <asmwarrior@gmail.com>

	* python/py-unwind.c (struct reg_info): Move out of ...
	(struct cached_frame_info): ... this scope.
	(pending_frame_object_type, unwind_info_object_type): Make extern.