Install systemd services with autotools

Change-Id: I103bf3468d53578045593eac31b61f7e0248495e
diff --git a/debian/osmo-sgsn.install b/debian/osmo-sgsn.install
index 9390deb..9d603f9 100644
--- a/debian/osmo-sgsn.install
+++ b/debian/osmo-sgsn.install
@@ -1,3 +1,4 @@
+lib/systemd/system/osmo-sgsn.service
 usr/bin/osmo-sgsn
 usr/share/doc/osmo-sgsn/examples/osmo-sgsn/osmo-sgsn.cfg usr/share/doc/osmo-sgsn/examples
 usr/share/doc/osmo-sgsn/examples/osmo-sgsn/osmo-sgsn-accept-all.cfg usr/share/doc/osmo-sgsn/examples