Use the GTest portability header in shell_test

Summary: It was including gtest directly, making MSVC unhappy.

Reviewed By: yfeldblum

Differential Revision: D4208744

fbshipit-source-id: 7100058f3c9fde38744888c99e927c610f11d013

folly/test/ShellTest.cpp

@@ -15,9 +15,9 @@
  */
 
 #include <glog/logging.h>
-#include <gtest/gtest.h>
 
 #include <folly/Shell.h>
+#include <folly/portability/GTest.h>
 
 using namespace folly;