Add the '=' action modifier that instructs tmpfiles.d to check the file
type of a path and remove objects that do not match before trying to
open or create the path.
BUG=chromium:1186405
TEST=./test/test-systemd-tmpfiles.py "$(which systemd-tmpfiles)"
Change-Id: If807dc0db427393e9e0047aba640d0d114897c26