[C++] linux-thread-db.c: dladdr cast

Fixes:

 src/gdb/linux-thread-db.c: In function ‘int try_thread_db_load_1(thread_db_info*)’:
 src/gdb/linux-thread-db.c:769:53: error: invalid conversion from ‘td_err_e (*)(ps_prochandle*, td_thragent_t**) {aka td_err_e (*)(ps_prochandle*, td_thragent**)}’ to ‘const void*’ [-fpermissive]
	library = dladdr_to_soname (*info->td_ta_new_p);
						      ^
 src/gdb/linux-thread-db.c:637:1: error:   initializing argument 1 of ‘const char* dladdr_to_soname(const void*)’ [-fpermissive]
  dladdr_to_soname (const void *addr)
  ^

gdb/ChangeLog:
2015-11-19  Pedro Alves  <palves@redhat.com>

	* linux-thread-db.c (try_thread_db_load_1): Add cast.