Fix a build failure

Summary: As title

Reviewed By: elsteveogrande

Differential Revision: D8643234

fbshipit-source-id: 989cdc53ed071fd78a1eba4b2955eff7651274f1

folly/experimental/observer/SimpleObservable-inl.h

@@ -63,7 +63,7 @@ struct SimpleObservable<T>::Wrapper {
   }
 
   void unsubscribe() {
-    folly::Function<void()> empty;
+    Function<void()> empty;
     context->callback_.swap(empty);
   }
 };