src: mark unused private field as such

Refs: https://github.com/nodejs/node/pull/59828
PR-URL: https://github.com/nodejs/node/pull/60802
Reviewed-By: Yagiz Nizipli <yagiz@nizipli.com>
Reviewed-By: Chengzhong Wu <legendecas@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>

src/node.h

@@ -1462,7 +1462,7 @@ class NODE_EXTERN CallbackScope {
   CallbackScope(CallbackScope&&) = delete;
 
  private:
-  void* resource_storage_global_;
+  [[maybe_unused]] void* reserved_;
   union {
     v8::Local<v8::Object> local;
     v8::Global<v8::Object>* global_ptr;