replaced mpd_malloc by G_GNUC_MALLOC

We want to remove gcc.h eventually. This takes care of all the
G_GNUC_MALLOC macros.

src/strset.c

@@ -47,7 +47,7 @@ static unsigned calc_hash(const char *p) {
 	return hash;
 }
 
-mpd_malloc struct strset *strset_new(void)
+G_GNUC_MALLOC struct strset *strset_new(void)
 {
 	struct strset *set = xcalloc(1, sizeof(*set));
 	return set;