webrtc/base: add dependent setting for gtest include directory that was missed when creating base_tests.gyp. Same as https://code.google.com/p/webrtc/source/browse/trunk/talk/libjingle_tests.gyp?r=6484#39

BUG=N/A
R=wu@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/16799004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@6574 4adac7df-926f-26a2-2b94-8c16560cd09d