Enable the -Wundef warning for clang

NOPRESUBMIT=true
BUG=webrtc:6398

Review-Url: https://codereview.webrtc.org/2358993004
Cr-Commit-Position: refs/heads/master@{#14425}
diff --git a/webrtc/system_wrappers/source/stringize_macros_unittest.cc b/webrtc/system_wrappers/source/stringize_macros_unittest.cc
index c2f312b..8b103c5 100644
--- a/webrtc/system_wrappers/source/stringize_macros_unittest.cc
+++ b/webrtc/system_wrappers/source/stringize_macros_unittest.cc
@@ -10,7 +10,7 @@
 
 #include "webrtc/system_wrappers/include/stringize_macros.h"
 
-#include "testing/gtest/include/gtest/gtest.h"
+#include "webrtc/test/gtest.h"
 
 // Macros as per documentation in header file.
 #define PREPROCESSOR_UTIL_UNITTEST_A FOO