main/postmarketos-ui-mate: fix installation of -openrc subpackage

MR https://gitlab.com/postmarketOS/pmaports/-/merge_requests/5374 created -openrc subpackages for postmarketOS user interface packages. In the MATE UI package APKBUILD, the installation line to draw in the -openrc subpackage is missing an "_if". Therefore the subpackage postmarketos-ui-mate-openrc doesn't get installed, the according OpenRC services don't get added and won't be started at boot. The graphical environment doesn't start, networking fails, etc. Also the username for autologin doesn't get changed in file /usr/share/lightdm/lightdm.conf.d/61-autologin.conf.

This MR adds the missing "_if".