systemd/systemd-services: add rtkit-daemon.service

This requires CONFIG_RT_GROUP_SCHED to be turned off in kernel configs to work properly.

We might also add rtkit as a dependency to install it by default, as when testing with gnome-mobile i had to manually install it.

See:

• https://bugzilla.redhat.com/show_bug.cgi?id=1229700
• https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1284731