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/aligned_array_unittest.cc b/webrtc/system_wrappers/source/aligned_array_unittest.cc
index 01238f8..5f3cdf7 100644
--- a/webrtc/system_wrappers/source/aligned_array_unittest.cc
+++ b/webrtc/system_wrappers/source/aligned_array_unittest.cc
@@ -12,7 +12,7 @@
 
 #include <stdint.h>
 
-#include "testing/gtest/include/gtest/gtest.h"
+#include "webrtc/test/gtest.h"
 
 namespace {