create pmg-scores.cf unconditionally

create pmg-scores.cf unconditionally

with the recent addtion of adjustable SA-rule scores, we introduced
'/etc/mail/spamassassin/pmg-scores.cf' as a new file and included it in the
cluster synchronization.

If the file does not exist the `rsync` command complains leading to misleading
errors in the journal.

Unconditionally creating the file (like we do for the already existing
'custom.cf') removes the warnings.

Reported-by: Martin Maurer <martin@proxmox.com>
Signed-off-by: Stoiko Ivanov <s.ivanov@proxmox.com>