Force the call of systemd-journal-flush before `/var` unmount

The problem with failed `/var` unmount placed on separate partition
have a long story: https://github.com/systemd/systemd/issues/867
According messages after the fix integrated into upstream -- there are
some corner cases which aren't fully fixed by patches adding [relinquish
options](pkg/systemd!26

)

In case of ostree-based images we have a similar issue since `/var` is
bind-mounted in initramfs. Systemd is trying to unmount `/var` on
shutdown, however `ExecStop=` command from `systemd-journal-flush`
service is not executed during shutdown.
By adding `PartOf=var.mount` into service file we force the
`systemd-journal-flush.service` to be called prior the `/var` unmount.
This allow to unlock the bind-mount, since `journald` have a chance to
re-link it's journal into `/run` with `journalctl
--smart-relinquish-var` call.

Signed-off-by: Denis Pynkin <denis.pynkin@collabora.com>