systemd: Do not restart when the application exits with 1

This is the case of a wrong config or failure to create a tunnel
diff --git a/contrib/openggsn.service b/contrib/openggsn.service
index 3b7d8cb..82f953a 100644
--- a/contrib/openggsn.service
+++ b/contrib/openggsn.service
@@ -6,3 +6,4 @@
 Restart=always
 ExecStart=/usr/bin/ggsn -c /etc/ggsn.conf -f
 RestartSec=2
+RestartPreventExitStatus=1