contrib: osmo-ggsn.service: Use expected suffix for cfg file

files in doc/examples/ dir end with .cfg, as well as all configuration
files present in all projects.

Change-Id: I361c67809d095dd08b0f400de2a6f84f981411c5
diff --git a/contrib/osmo-ggsn.service b/contrib/osmo-ggsn.service
index a8d123a..10e3507 100644
--- a/contrib/osmo-ggsn.service
+++ b/contrib/osmo-ggsn.service
@@ -5,7 +5,7 @@
 [Service]
 Type=simple
 Restart=always
-ExecStart=/usr/bin/osmo-ggsn -c /etc/osmocom/osmo-ggsn.conf -f
+ExecStart=/usr/bin/osmo-ggsn -c /etc/osmocom/osmo-ggsn.cfg -f
 RestartSec=2
 RestartPreventExitStatus=1