liblxc1: add depends on cgroupfs-mount | systemd

We need something that mounts cgroups for us.

Usually, this will be systemd, but sometimes people don't run systemd
as pid1. Add cgroupfs-mount for this case.

As systemd is installed by default, this dependency will be mostly noop.
But when systemd is not installed, prefer cgroupfs-mount as people
probably don't have systemd installed on purpose.

Closes: #844086