Add cast to exception_none

Add cast to exception_none

Fixes, in C++ mode:

 ../../src/gdb/common/common-exceptions.c:23:69: error: invalid conversion from ‘int’ to ‘return_reason’ [-fpermissive]
  const struct gdb_exception exception_none = { 0, GDB_NO_ERROR, NULL };
								      ^

(I considered adding an enum value for '0', but the code and comments
around return_reason and its uses explain how 0 is special/internal,
so I'm leaving it be.)

gdb/ChangeLog:
2015-10-29  Pedro Alves  <palves@redhat.com>

	* common/common-exceptions.c (exception_none): Add cast.