Added forward declaration of ObserverCreator

Summary:
For some reason, compilation of certain Observable code fails in opt mode with the error that Observer::core_ is private.

It seems that ObserverCreator is not recognized as a friend of Observer. This forward declaration of ObserverCreator seems to fix this issue.

Reviewed By: yfeldblum

Differential Revision: D7268318

fbshipit-source-id: 1ea76efb0f50cb8a6b28147305ec7547f55338b8

folly/experimental/observer/Observer.h

@@ -140,6 +140,9 @@ class CallbackHandle {
   std::shared_ptr<Context> context_;
 };
 
+template <typename Observable, typename Traits>
+class ObserverCreator;
+
 template <typename T>
 class Observer {
  public: