Enable the -Wundef warning for clang

webrtc/modules/video_capture/test/video_capture_main_mac.mm

Index: webrtc/modules/video_capture/test/video_capture_main_mac.mm
diff --git a/webrtc/modules/video_capture/test/video_capture_main_mac.mm b/webrtc/modules/video_capture/test/video_capture_main_mac.mm
index 15cd1d555fd1cee7430c00a6d646bf04a027c58e..6d6d5c64792a5a96d16e47bd334dc462a5ea82f0 100644
--- a/webrtc/modules/video_capture/test/video_capture_main_mac.mm
+++ b/webrtc/modules/video_capture/test/video_capture_main_mac.mm
@@ -8,7 +8,7 @@
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
-#include "testing/gtest/include/gtest/gtest.h"
+#include "webrtc/test/gtest.h"
 #include "webrtc/test/testsupport/mac/run_threaded_main_mac.h"
 
 int ImplementThisToRunYourTest(int argc, char** argv) {