Add p2p.gyp to rtc_base presubmit check exclusion.

It is one of the valid users of rtc_base. This change
should have been a part of https://webrtc-codereview.appspot.com/29319004

TESTED=Modified p2p.gyp locally and ran the presubmit without+with this
patch applied. Only the latter passed.

R=pbos@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/43449004

Cr-Commit-Position: refs/heads/master@{#8511}
git-svn-id: http://webrtc.googlecode.com/svn/trunk@8511 4adac7df-926f-26a2-2b94-8c16560cd09d
1 file changed