units: add dep on systemd-logind.service by user@.service

units: add dep on systemd-logind.service by user@.service

Let's make sure logind is accessible by the time user@.service runs, and
that logind stays around as long as it does so.

Addresses an issue reported here:

https://lists.freedesktop.org/archives/systemd-devel/2024-June/050468.html

This addresses an issued introduced by
278e815bfa3e4c2e3914e00121c37fc844cb2025, which dropped the a dependency
from user@.service systemd-user-sessions.service without replacement.
While dropping that dependency does make sense, it should have been
replaced with the weaker dependency on systemd-logind.service, hence fix
that now.

user@.service is after all a logind concept, hence logind really should
be around for its lifetime.

systemd-user-sessions.service is a later milestone that only really
should apply to regular users (not root), hence it's too strong a
requirement.