Fix compilation error folly/concurrency/test/AtomicSharedPtrTest.cpp (#1464)

Summary:
Hello,
It seems it is a missing include.
Regards,
Laurent

Pull Request resolved: https://github.com/facebook/folly/pull/1464

Reviewed By: yfeldblum

Differential Revision: D25730356

Pulled By: Orvid

fbshipit-source-id: 61faa2fdc161450909de6678c1bb8aa331260f64

folly/concurrency/test/AtomicSharedPtrTest.cpp

@@ -27,6 +27,7 @@
 #include <thread>
 
 #include <folly/concurrency/AtomicSharedPtr.h>
+#include <folly/portability/GFlags.h>
 #include <folly/portability/GTest.h>
 
 #include <folly/test/DeterministicSchedule.h>