openvpn: update to v2.4.2

Update to version 2.4.2 in order to address two potential Denial-of-Service
vectors in OpenVPN.

CVE-2017-7478 - Don't assert out on receiving too-large control packets
CVE-2017-7479 - Drop packets instead of assert out if packet id rolls over

Ref: https://community.openvpn.net/openvpn/wiki/ChangesInOpenvpn24#OpenVPN2.4.2
Ref: https://community.openvpn.net/openvpn/wiki/QuarkslabAndCryptographyEngineerAudits



Signed-off-by: Jo-Philipp Wich <jo@mein.io>