if_ovpn tests: fix WITHOUT_PF

At least one if_ovpn.sh test relies on pf, so the file includes
netpfil/pf/utils.subr, which doesn't exist if WITHOUT_PF is set.

Do not install the if_ovpn.sh tests if pf is disabled.

Suggested by: Olivier Cochard-Labbé <olivier@freebsd.org>
Sponsored by: Rubicon Communications, LLC ("Netgate")