Install sample cfg file to /etc/osmocom

Change-Id: I6926e989a130086f4b6c8277407377a4063f452f
diff --git a/debian/osmo-mgw.install b/debian/osmo-mgw.install
index ad3d3c6..6f44961 100644
--- a/debian/osmo-mgw.install
+++ b/debian/osmo-mgw.install
@@ -1,3 +1,4 @@
+etc/osmocom/osmo-mgw.cfg
 lib/systemd/system/osmo-mgw.service
 usr/bin/osmo-mgw
 usr/share/doc/osmo-mgw/examples/osmo-mgw/osmo-mgw.cfg