Install sample cfg file to /etc/osmocom

Change-Id: Id898e920b934fbfb0b9abe1e3085ac817fa4634d
diff --git a/debian/osmo-sgsn.install b/debian/osmo-sgsn.install
index 9d603f9..b9a46ab 100644
--- a/debian/osmo-sgsn.install
+++ b/debian/osmo-sgsn.install
@@ -1,3 +1,4 @@
+/etc/osmocom/osmo-sgsn.cfg
 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