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

tmpfiles.d/var.conf.m4

@@ -15,6 +15,7 @@ d /var/log 0755 - - -
 m4_ifdef(`HAVE_UTMP',
 f /var/log/wtmp 0664 root utmp -
 f /var/log/btmp 0600 root utmp -
+f /var/log/lastlog 0664 root utmp -
 )m4_dnl
 
 d /var/cache 0755 - - -