debian: include systemd service osmo-mgw.service

Change-Id: Ic298b836620b3497734aed89b92c4f22a9071f0d
diff --git a/debian/osmo-mgw.service b/debian/osmo-mgw.service
new file mode 120000
index 0000000..1e0664c
--- /dev/null
+++ b/debian/osmo-mgw.service
@@ -0,0 +1 @@
+../contrib/systemd/osmo-mgw.service
\ No newline at end of file