fix for phosh under gnome-session 46

Upstream PR: https://gitlab.gnome.org/World/Phosh/phosh/-/merge_requests/1391

Without this patch, phosh can no longer start as gnome-session 46 has dropped the --systemd flag and will fail if it is passed.

It's already being applied to Mobian so I think the patch is already in good shape: https://salsa.debian.org/DebianOnMobile-team/phosh/-/merge_requests/74/diffs

I also added python-packaging to makedepends because I couldn't get a clean build in chroot without it.

I've tested this patch locally and confirmed it fixes phosh under gnome-session 46.