References to includes in third_party should be relative, not absolute.

BUG=
R=henrik.lundin@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/9989004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@5704 4adac7df-926f-26a2-2b94-8c16560cd09d