backup: add SA custom score file to backup

backup: add SA custom score file to backup

/etc/mail/spamassassin/pmg-scores.cf contains the custom scores users can
set via GUI. It should be included in the backup (and restored if present).

Reported via our community forum:
https://forum.proxmox.com/threads/backup-restore-bug.78605/

Tested on my local setup by creating a bogus score, creating a backup,
removing the pmg-scores file and restoring the backup.

Signed-off-by: Stoiko Ivanov <s.ivanov@proxmox.com>