string.c: suppress a warning

* string.c (rb_str_enumerate_grapheme_clusters): suppress a
  maybe-uninitialized warning by old gcc.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@59730 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

string.c

@@ -8090,7 +8090,7 @@ rb_str_enumerate_grapheme_clusters(VALUE str, int wantarray)
     rb_encoding *enc = rb_enc_from_index(encidx);
     int unicode_p = rb_enc_unicode_p(enc);
     const char *ptr, *end;
-    VALUE ary;
+    VALUE MAYBE_UNUSED(ary);
 
     if (!unicode_p) {
 	return rb_str_enumerate_codepoints(str, wantarray);