src: add missing override specifier to Clean()

The `CallbackInfo::Clean()` method overrides `Cleanable::Clean()` and
should be marked as `override`, both because it is good practice to do
so and for consistency with the rest of the code base.

Author: tniessen

src/node_buffer.cc

@@ -98,7 +98,7 @@ class CallbackInfo : public Cleanable {
   CallbackInfo& operator=(const CallbackInfo&) = delete;
 
  private:
-  void Clean();
+  void Clean() override;
   inline void OnBackingStoreFree();
   inline void CallAndResetCallback();
   inline CallbackInfo(Environment* env,