postfix: Add missing smtp-ipv4 service

Signed-off-by: Florian Pritz <bluewind@xinu.at>

roles/postfix/templates/master.cf.j2

@@ -66,6 +66,8 @@ flush     unix  n       -       n       1000?   0       flush
 proxymap  unix  -       -       n       -       -       proxymap
 proxywrite unix -       -       n       -       1       proxymap
 smtp      unix  -       -       n       -       -       smtp
+smtp-ipv4      unix  -       -       n       -       -       smtp
+ -o inet_protocols=ipv4
 relay     unix  -       -       n       -       -       smtp
 #       -o smtp_helo_timeout=5 -o smtp_connect_timeout=5
 showq     unix  n       -       n       -       -       showq