security/mailzu: Fix support for PHP 7.x

[joneum]

Summary

Hey Guys,

can you please take a look at this review. It fixes the support for PHP 7.x.
The new maintainer has already spoken here with tz. Please send me feedback on this update.
Thank you :-)

PR: 230712

Test Plan

11.2:
http://joneumbox.org/build.html?mastername=112amd64-ports&build=2018-11-11_13h52m46s
http://joneumbox.org/build.html?mastername=112i386-ports&build=2018-11-11_13h52m46s

12.0
http://joneumbox.org/build.html?mastername=12amd64-ports&build=2018-11-11_14h19m55s
http://joneumbox.org/build.html?mastername=12i386-ports&build=2018-11-11_14h20m03s

13-CURRENT
http://joneumbox.org/build.html?mastername=13amd64-ports&build=2018-11-11_14h56m02s
http://joneumbox.org/build.html?mastername=13i386-ports&build=2018-11-11_14h59m15s

[tcberner]

Wouldn't phpflavors make sense here?