main/postmarketos-base-ui-gnome: enable apk-polkit-rs service

Needed by GNOME Software since https://gitlab.alpinelinux.org/Cogitri/apk-polkit-rs/-/merge_requests/45

GNOME Software is often installed in GNOME UI's through pmb_recommends, but it's not a dependency in any way. To avoid spilling this service enablement everywhere, add just the openrc subpackage from the daemon. That means that if GNOME Software is not installed or gets uninstalled, the daemon will crash on startup. That should not be a big deal though.