experimental/observer/detail/ObserverManager.cpp missing gflags

Summary: Fix the build due to a missing gflags header

Reviewed By: yfeldblum

Differential Revision: D3905525

fbshipit-source-id: cc09189196a04741162c2d1561545d3fec417e5e

folly/experimental/observer/detail/ObserverManager.cpp

@@ -17,6 +17,7 @@
 
 #include <folly/MPMCQueue.h>
 #include <folly/Singleton.h>
+#include <folly/portability/GFlags.h>
 
 namespace folly {
 namespace observer_detail {