Install sample cfg file to /etc/osmocom

Change-Id: If41e69295ac23a61df138ceea83794059f111086
diff --git a/debian/osmo-ggsn.install b/debian/osmo-ggsn.install
index 17caef7..35724d2 100644
--- a/debian/osmo-ggsn.install
+++ b/debian/osmo-ggsn.install
@@ -1,3 +1,4 @@
+/etc/osmocom/osmo-ggsn.cfg
 /lib/systemd/system/osmo-ggsn.service
 /usr/bin/osmo-ggsn
 /usr/bin/sgsnemu