Use $postfix::_smtp_listen in files

manifests/files.pp

@@ -9,7 +9,7 @@ class postfix::files {
   $master_submission = $postfix::master_submission
   $myorigin = $postfix::myorigin
   $root_mail_recipient = $postfix::root_mail_recipient
-  $smtp_listen = $postfix::smtp_listen
+  $smtp_listen = $postfix::_smtp_listen
   $use_amavisd = $postfix::use_amavisd
   $use_dovecot_lda = $postfix::use_dovecot_lda
   $use_schleuder = $postfix::use_schleuder