units: we need systemd-journald.service from systemd-journal-flush.service

units: we need systemd-journald.service from systemd-journal-flush.service

This is a follow-up for d5ee050ffc9d413253932d9340ade8c8fb111092, and
reintroduces a requirement dep from systemd-journal-flush.service onto
systemd-journald.service, but a weaker one than originally: a Wants= one
instead of a Requires= one.

Why? Simply because the service issues an IPC call to the journald,
hence it should pull it in. (Note that socket activation doesn't happen
for the Varlink socket it uses, hence we should pull in the service
itself.)