nullmailer: Fix queue service installation

nullmailer/files/nullmailer-queued.service

 [Unit]
 Description=Enqueueing facility for restricted clients of nullmailer
 After=networking.target
-Before=php-fpm@.service
 
 [Service]
 ExecStart=/usr/local/sbin/nullmailer-queued
@@ -11,3 +10,6 @@ ProtectSystem=strict
 ProtectControlGroups=yes
 ProtectHome=yes
 PrivateTmp=yes
+
+[Install]
+WantedBy=multi-user.target