fixed several gcc warnings on unused debug variables

src/decoder_api.c

@@ -355,7 +355,7 @@ enum decoder_command
 decoder_tag(G_GNUC_UNUSED struct decoder *decoder, struct input_stream *is,
 	    const struct tag *tag)
 {
-	const struct decoder_control *dc = decoder->dc;
+	G_GNUC_UNUSED const struct decoder_control *dc = decoder->dc;
 	enum decoder_command cmd;
 
 	assert(dc->state == DECODE_STATE_DECODE);

src/inotify_update.c

@@ -80,6 +80,7 @@ tree_add_watch_directory(struct watch_directory *directory)
 static void
 tree_remove_watch_directory(struct watch_directory *directory)
 {
+	G_GNUC_UNUSED
 	bool found = g_tree_remove(inotify_directories,
 				   GINT_TO_POINTER(directory->descriptor));
 	assert(found);

src/player_thread.c

@@ -426,7 +426,7 @@ static bool player_seek_decoder(struct player *player)
  */
 static void player_process_command(struct player *player)
 {
-	struct decoder_control *dc = player->dc;
+	G_GNUC_UNUSED struct decoder_control *dc = player->dc;
 
 	switch (pc.command) {
 	case PLAYER_COMMAND_NONE: