Call dh_installsysusers explicitly, Depend on systemd-sysusers

dh_installsysusers generates relevant integration snippets for enabling the users on installation. Without it, the user 'fwupd-refresh' is not created, and the service fails to start with:

systemd[1]: Starting fwupd-refresh.service - Refresh fwupd metadata and update motd...
(fwupdmgr)[3669]: fwupd-refresh.service: Failed to determine user credentials: No such process
systemd[1]: fwupd-refresh.service: Main process exited, code=exited, status=217/USER
systemd[1]: fwupd-refresh.service: Failed with result 'exit-code'.
systemd[1]: Failed to start fwupd-refresh.service - Refresh fwupd metadata and update motd.

The bug was introduced in commit 6e3af79c.

Closes: #1065466