units: Order user@.service after systemd-oomd.service

units: Order user@.service after systemd-oomd.service

The user manager connects to oomd over varlink. Currently, during
shutdown, if oomd is stopped before any user manager, the user
manager will try to reconnect to the socket, leading to a warning
from pid 1 about a conflicting transaction.

Let's fix this by ordering user@.service after systemd-oomd.service,
so that user sessions are stopped before systemd-oomd is stopped,
which makes sure that the user sessions won't try to start oomd via
its socket after systemd-oomd is stopped.