Use the gflags portability header

Summary: The gflags portability header should always be used in Folly. There were a couple of places gflags was being used directly. This fixes that.

Reviewed By: yfeldblum

Differential Revision: D4748674

fbshipit-source-id: ba3c0564a39f870f1f5f9371e32d06eb84591ba7

folly/experimental/hazptr/test/HazptrTest.cpp

@@ -21,7 +21,7 @@
 #include <folly/experimental/hazptr/debug.h>
 #include <folly/experimental/hazptr/hazptr.h>
 
-#include <gflags/gflags.h>
+#include <folly/portability/GFlags.h>
 #include <folly/portability/GTest.h>
 
 #include <thread>

folly/io/async/test/SocketClient.cpp

@@ -18,7 +18,7 @@
 #include <iostream>
 
 #include <folly/ExceptionWrapper.h>
-#include <gflags/gflags.h>
+#include <folly/portability/GFlags.h>
 
 using namespace folly;