security/strongswan: update 5.5.3 -> 5.6.0

• the gmp plugin responsible for CVE-2017-11185 is not enabled in the FreeBSD build

PR: 221716
Relnotes: https://wiki.strongswan.org/versions/66
Reported by: i.dani@outlook.com
Approved by: strongswan@nanoteq.com (maintainer)