enabled cpplint for the webrtc/modules/rtp_rtcp directory

BUG=webrtc:5277
R=kjellander@webrtc.org
NOTRY=True

Review URL: https://codereview.webrtc.org/1528863002

Cr-Commit-Position: refs/heads/master@{#11027}
diff --git a/PRESUBMIT.py b/PRESUBMIT.py
index 9db9d82..7557b6b 100755
--- a/PRESUBMIT.py
+++ b/PRESUBMIT.py
@@ -21,6 +21,7 @@
   'webrtc/common_video',
   'webrtc/examples',
   'webrtc/modules/remote_bitrate_estimator',
+  'webrtc/modules/rtp_rtcp',
   'webrtc/modules/video_processing',
   'webrtc/sound',
   'webrtc/tools',