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

@@ -1719,12 +1719,15 @@ mrb_str_reverse_bang(mrb_state *mrb, mrb_value str)
 
   if (RSTR_LEN(s) > 1) {
     mrb_str_modify(mrb, s);
-   bytes:
-    p = RSTR_PTR(s);
-    e = p + RSTR_LEN(s) - 1;
-    str_reverse(p, e);
+    goto bytes;
   }
   return str;
+
+ bytes:
+  p = RSTR_PTR(s);
+  e = p + RSTR_LEN(s) - 1;
+  str_reverse(p, e);
+  return str;
 }
 
 /* ---------------------------------- */