mail/postfixadmin: Update to 3.1

Ensure emplates_c/ directory is created.

Changes: https://github.com/postfixadmin/postfixadmin/blob/master/CHANGELOG.TXT

PR: 221965
Submitted by: Lukasz Wasikowski <lukasz@wasikowski.net>
Approved by: Melissa Pilgrim <ports.maintainer@evilphi.com> (maintainer)