Install systemd services with autotools

Change-Id: I90f1efc4433ec641a28931d5c19e2301a67cf1e9
diff --git a/debian/osmo-hnbgw.install b/debian/osmo-hnbgw.install
index e230915..bb70ad5 100644
--- a/debian/osmo-hnbgw.install
+++ b/debian/osmo-hnbgw.install
@@ -1,2 +1,3 @@
+lib/systemd/system/osmo-hnbgw.service
 usr/bin/osmo-hnbgw
 usr/share/doc/osmo-iuh/examples/osmo-hnbgw.cfg