Reorder members of `struct os_each_object_data` to stop warnings.

`mrb_value` may or may not be struct according to configuration.

mrbgems/mruby-objectspace/src/mruby_objectspace.c

@@ -112,9 +112,9 @@ os_count_objects(mrb_state *mrb, mrb_value self)
 }
 
 struct os_each_object_data {
-  mrb_value block;
   struct RClass *target_module;
   mrb_int count;
+  mrb_value block;
 };
 
 static int