tag/TagPool: use gcc_packed instead of mpd_packed

By accident, this declared a global variable instead of adding the
"packed" attribute.

src/tag/TagPool.cxx

@@ -35,7 +35,7 @@ struct slot {
 	struct slot *next;
 	unsigned char ref;
 	TagItem item;
-} mpd_packed;
+} gcc_packed;
 
 static struct slot *slots[NUM_SLOTS];