Silence unused label warnings from gcc; ref #4524

  mruby/mruby/src/string.c:1722:4: warning: label 'bytes' defined but not used [-Wunused-label]
      bytes:
      ^~~~~

src/string.c

@@ -1719,11 +1719,14 @@ mrb_str_reverse_bang(mrb_state *mrb, mrb_value str)
 
   if (RSTR_LEN(s) > 1) {
     mrb_str_modify(mrb, s);
+    goto bytes;
+  }
+  return str;
+
  bytes:
   p = RSTR_PTR(s);
   e = p + RSTR_LEN(s) - 1;
   str_reverse(p, e);
-  }
   return str;
 }