Fix signed/unsigned warning.

mrbgems/mruby-struct/src/struct.c

@@ -185,8 +185,7 @@ static mrb_value
 mrb_struct_set(mrb_state *mrb, mrb_value obj, mrb_value val)
 {
   const char *name;
-  int i;
-  size_t len;
+  size_t i, len;
   mrb_sym mid;
   mrb_value members, slot, *ptr, *ptr_members;