[C++/mingw] gdb-dlfcn.c casts

[C++/mingw] gdb-dlfcn.c casts

Fixes:

../../src/gdb/gdb-dlfcn.c: In function 'void* gdb_dlsym(void*, const char*)':
../../src/gdb/gdb-dlfcn.c:105:49: error: invalid conversion from 'void*' to 'HMODULE {aka HINSTANCE__*}' [-fpermissive]
   return (void *) GetProcAddress (handle, symbol);
                                                 ^

gdb/ChangeLog:
2015-11-17  Pedro Alves  <palves@redhat.com>

	* gdb-dlfcn.c (gdb_dlsym, gdb_dlclose) [__MINGW32__]: Add casts to
	HMODULE.