add dummy ss7 service dumper module

Instead of implementing a real subsystem, it just dumps all MTP3 payload
messages of specified service
diff --git a/ebin/osmo_ss7.app b/ebin/osmo_ss7.app
index 20f143f..f3d5f05 100644
--- a/ebin/osmo_ss7.app
+++ b/ebin/osmo_ss7.app
@@ -11,6 +11,7 @@
 			sccp_codec,
 			osmo_ss7_sup, osmo_ss7_app,
 			ss7_links, ss7_link_m3ua, ss7_link_ipa_client,
+			ss7_service_dump,
 			osmo_ss7_gtt,
 			osmo_ss7_pcap
 		]},