Include systemd units in the source tarball (make dist)

Acked-by: David Sommerseth <davids@redhat.com>
Message-Id: 1417548862-24990-1-git-send-email-floppym@gentoo.org
URL: http://article.gmane.org/gmane.network.openvpn.devel/9299
Signed-off-by: David Sommerseth <davids@redhat.com>
(cherry picked from commit 6ece60c6dc7a3cda58f4dfea4e6cd3016023234f)