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
diff --git a/PRESUBMIT.py b/PRESUBMIT.py
index 81196c9..381119e 100755
--- a/PRESUBMIT.py
+++ b/PRESUBMIT.py
@@ -106,6 +106,7 @@
         'desktop_capture.gypi',
         'libjingle.gyp',
         'libjingle_tests.gyp',
+        'p2p.gyp',
         'sound.gyp',
         'webrtc_test_common.gyp',
         'webrtc_tests.gypi',