add check_policy_service only when postgrey set to true

[efoft]

Currently check_policy_service unix:postgrey/socket is always added to smtpd_recipient_restrictions independently of what $postgrey is set (provided smtpd_recipient_restrictions is not empty). Added check into erb templates.

[rmc47]

@thias I don't suppose there'd be any chance of a merge on this, would there? Hitting the same issue, and the fix looks good.