build: ensure we do not fail clean if check-results.xml does not exist
build: ensure we do not fail clean if check-results.xml does not exist

If we did not run make check this file would not exist and a clean would
fail over it. Using -f ignores this case.

rvandegrift/elementary,rvandegrift/elementary,tasn/elementary,tasn/elementary,tasn/elementary,tasn/elementary,rvandegrift/elementary,rvandegrift/elementary,tasn/elementary