units: make systemd-journald.service Type=notify

It already calls sd_notify(), so it looks like an oversight.

Without it, its ordering to systemd-journal-flush.service is
non-deterministic and the SIGUSR1 from flushing may kill journald before
it has its signal handlers set up.

https://bugs.freedesktop.org/show_bug.cgi?id=85871
https://bugzilla.redhat.com/show_bug.cgi?id=1159641