openggsn.service: Add Install section to systemd service file

This way the service can be enabled through
> systemctl enable openggsn
diff --git a/contrib/openggsn.service b/contrib/openggsn.service
index 82f953a..35301cd 100644
--- a/contrib/openggsn.service
+++ b/contrib/openggsn.service
@@ -7,3 +7,6 @@
 ExecStart=/usr/bin/ggsn -c /etc/ggsn.conf -f
 RestartSec=2
 RestartPreventExitStatus=1
+
+[Install]
+WantedBy=multi-user.target