Filter out messages from included files

Summary:
CppCheck shows lint messages from included files as well as the current
file. Filter out those, since they don't make much sense in the context
of `arc lint`.

Test Plan:
Before this patch, `arc lint` using `ArcanistCppcheckLinter`. Note that lint messages
from included files appear pointing to a line in the active file.
After this patch, only messages from the active file are included.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley

CC: Korvin, epriestley, aran

Differential Revision: https://secure.phabricator.com/D7444