systemd service: add save_log_tail

Change-Id: I6f65e3d774197ca82cfda62e3a32f8fbf044e449
diff --git a/contrib/systemd/osmo-ggsn.service b/contrib/systemd/osmo-ggsn.service
index c75325d..b368201 100644
--- a/contrib/systemd/osmo-ggsn.service
+++ b/contrib/systemd/osmo-ggsn.service
@@ -8,6 +8,7 @@
 ExecStart=/usr/bin/osmo-ggsn -c /etc/osmocom/osmo-ggsn.cfg
 RestartSec=2
 RestartPreventExitStatus=1
+ExecStopPost=/usr/local/bin/save_log_tail osmo-ggsn
 
 [Install]
 WantedBy=multi-user.target