tmpfiles: create /var/log/lastlog if it does not exist

Create /var/log/lastlog the same way we create utmp and wtmp.
This is useful for stateless systems where /var is volatile and a
missing /var/log/lastlog otherwise creates error messages like

Jun 27 20:00:00 huron sshd[1234]: lastlog_openseek: Couldn't stat /var/log/lastlog: No such file or directory

Fixes #6234