bfd/libhppa.h: Make C++ compatible

bfd/libhppa.h: Make C++ compatible

Fixes this when GDB is built in C++ mode:

 In file included from /home/pedro/gdb/mygit/src/gdb/../bfd/som.h:27:0,
		  from /home/pedro/gdb/mygit/src/gdb/somread.c:31:
 /home/pedro/gdb/mygit/src/gdb/../bfd/libhppa.h: In function ‘int bfd_hppa_insn2fmt(bfd*, int)’:
 /home/pedro/gdb/mygit/src/gdb/../bfd/libhppa.h:380:42: error: invalid conversion from ‘int’ to ‘hppa_opcode_type’ [-fpermissive]
  #define get_opcode(insn) (((insn) >> 26) & 0x3f)
					   ^
 /home/pedro/gdb/mygit/src/gdb/../bfd/libhppa.h:465:30: note: in expansion of macro ‘get_opcode’
    enum hppa_opcode_type op = get_opcode (insn);
			       ^

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

	* libhppa.h (bfd_hppa_insn2fmt): Add cast.