.gitignore: Add *.mk, created as part of ChromiumOS build

Contributed by Josh Triplett <josh.triplett@intel.com>

BUG=None
TEST=Build Chromium and ChromiumOS from source, and run "repo status",
     with and without this change.

Review URL: https://webrtc-codereview.appspot.com/1000006

git-svn-id: http://webrtc.googlecode.com/svn/trunk@3334 4adac7df-926f-26a2-2b94-8c16560cd09d
2 files changed