systemd: Add service for the osmo-sgsn

Fixes: SYS#175
diff --git a/openbsc/contrib/systemd/osmo-sgsn.service b/openbsc/contrib/systemd/osmo-sgsn.service
new file mode 100644
index 0000000..afd0f40
--- /dev/null
+++ b/openbsc/contrib/systemd/osmo-sgsn.service
@@ -0,0 +1,8 @@
+[Unit]
+Description=OpenBSC SGSN
+
+[Service]
+Type=simple
+Restart=always
+ExecStart=/usr/bin/osmo-sgsn -c /etc/osmocom/osmo-sgsn.cfg
+RestartSec=2