gnome-session: enable systemd session support + phosh: enable systemd integration

This behaviour is no longer configurable with GNOME Session 46 and only what's being configured here is available.

All that happens is that GNOME Session fails to find systemd and uses the fallback behaviour. The only difference is that if it were to somehow find systemd it would now use it, whereas before it would always use the fallback behaviour. So in practice, not much difference.

I did a quick test on my Librem 5 with Phosh and elogind and it works.