include gflag explicitly

Summary:
fix #839
Closes https://github.com/facebook/folly/pull/846

Reviewed By: yfeldblum

Differential Revision: D7987128

Pulled By: Orvid

fbshipit-source-id: 626ed1724f3df5fecc83413574cfd72f1f0b1fa1

folly/executors/ThreadPoolExecutor.h

@@ -21,6 +21,7 @@
 #include <folly/executors/task_queue/LifoSemMPMCQueue.h>
 #include <folly/executors/thread_factory/NamedThreadFactory.h>
 #include <folly/io/async/Request.h>
+#include <folly/portability/GFlags.h>
 #include <folly/synchronization/Baton.h>
 
 #include <algorithm>