update: fix deadlock in delete_song()

Due to a merge error, reap_update_task() called cond_signal_async()
with a locked mutex.  That always fails.  Use cond_signal_sync()
instead.

src/update.c

@@ -525,7 +525,7 @@ void reap_update_task(void)
 		LOG("removing: %s\n", song_get_url(delete, tmp));
 		deleteASongFromPlaylist(delete);
 		delete = NULL;
-		cond_signal_async(&delete_cond);
+		cond_signal_sync(&delete_cond);
 	}
 	cond_leave(&delete_cond);