Add missing includes of folly/synchronization/Baton.h

Summary: Add missing includes of `folly/synchronization/Baton.h`.

Differential Revision: D21716396

fbshipit-source-id: 8c8fb818f3083b4fe04fd1c62923d7ba698dcedd

folly/experimental/observer/Observable.h

@@ -17,6 +17,7 @@
 #pragma once
 
 #include <folly/experimental/observer/Observer.h>
+#include <folly/synchronization/Baton.h>
 
 namespace folly {
 namespace observer {

folly/io/async/test/EventBaseTestLib.h

@@ -27,6 +27,7 @@
 #include <folly/system/ThreadName.h>
 
 #include <folly/futures/Promise.h>
+#include <folly/synchronization/Baton.h>
 
 #include <atomic>
 #include <future>