security/openvpn: Update to bugfix release 2.7.4

Remove --enable-strict option, now unsupported upstream

Changelog: https://github.com/OpenVPN/openvpn/blob/v2.7.4/Changes.rst

Obtained from: GitHub repo

(cherry picked from commit 68035ac676e92b0eeb2965baaae02b6f6d676c11)