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/api/peerconnectioninterface_unittest.cc b/webrtc/api/peerconnectioninterface_unittest.cc
index d9c8af0..c639f98 100644
--- a/webrtc/api/peerconnectioninterface_unittest.cc
+++ b/webrtc/api/peerconnectioninterface_unittest.cc
@@ -12,7 +12,7 @@
 #include <string>
 #include <utility>
 
-#include "testing/gmock/include/gmock/gmock.h"
+#include "webrtc/test/gmock.h"
 #include "webrtc/api/audiotrack.h"
 #include "webrtc/api/jsepsessiondescription.h"
 #include "webrtc/api/mediastream.h"