gdbarch.h: include regcache.h

gdbarch.h: include regcache.h

Building GDB in C++ mode, I got:

  src/gdb/gdbarch.h:240:149: error: invalid type in declaration before ‘;’ token
  src/gdb/gdbarch.h:240:14: error: use of enum ‘register_status’ without previous declaration
  src/gdb/gdbarch.h:241:13: error: use of enum ‘register_status’ without previous declaration
  src/gdb/gdbarch.h:241:140: error: invalid type in declaration before ‘;’ token

That's because 'enum register_status' has not been declared (and we can't
forward declare enums in C++).

gdb/ChangeLog:
2015-02-27  Pedro Alves  <palves@redhat.com>

	* gdbarch.sh: Include regcache.h.
	* gdbarch.h: Regenerate.