Swap a newly added include of gtest.h with portability/GTest.h

Summary: Because it's needed with the way the portability layer works.

Reviewed By: yfeldblum

Differential Revision: D3863954

fbshipit-source-id: 3b682dcc2d2799f17c61fe16a4391fc9f0c2b127

folly/test/SingletonThreadLocalTest.cpp

@@ -20,8 +20,7 @@
 
 #include <folly/SingletonThreadLocal.h>
 #include <folly/Synchronized.h>
-
-#include <gtest/gtest.h>
+#include <folly/portability/GTest.h>
 
 using namespace folly;