Also restrict include/ rules to only add bots for changes affecting
headers

R=machenbach@chromium.org

Change-Id: I97dc3c6249994566912a82ac9fbf8c7d3a57890c
Reviewed-on: https://chromium-review.googlesource.com/519342Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Jochen Eisinger <jochen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45638}