contrib: Start openggsn after networking

Change-Id: Ic617c618db17e7109ee864c4e6c86f3bee081d02
diff --git a/contrib/openggsn.service b/contrib/openggsn.service
index 35301cd..a34f374 100644
--- a/contrib/openggsn.service
+++ b/contrib/openggsn.service
@@ -1,5 +1,6 @@
 [Unit]
 Description=OpenGGSN
+After=networking.service
 
 [Service]
 Type=simple