gdb: Add missing #ifdef USE_THREAD_DB to gdbserver
    
    Otherwise, linking fails with:
    
      [...]/linux-low.c:664: undefined reference to `thread_db_notice_clone(thread_info*, ptid_t)'
    
    gdb/gdbserver/ChangeLog:
    
    	* linux-low.c (handle_extended_wait): Surround call to
    	thread_db_notice_clone with #ifdef USE_THREAD_DB.