Disable HarbTabs check for Makefiles

These require hard tabs, so it makes sense to skip the tab-enforcement.

Change-Id: I4411d42231e2cbba13ad6c5b0d19e9fa5bed65f6
Reviewed-on: http://gerrit.causes.com/40164
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Joe Lencioni <joe.lencioni@brigade.com>