D-GSM 2/n: implement mDNS method of mslookup server

Implement the mslookup server's mDNS responder, to actually service remote
mslookup requests:
- VTY mslookup/server config with service names,
- the mslookup_mdns_server listening for mslookup requests,

For a detailed overview of the D-GSM and mslookup related files, please see the
elaborate comment at the top of mslookup.c (already added in an earlier patch).

Change-Id: I5cae6459090588b4dd292be90a5e8903432669d2
diff --git a/include/osmocom/hlr/mslookup_server.h b/include/osmocom/hlr/mslookup_server.h
index 5425328..93b419d 100644
--- a/include/osmocom/hlr/mslookup_server.h
+++ b/include/osmocom/hlr/mslookup_server.h
@@ -19,6 +19,9 @@
 
 #pragma once
 
+#include <osmocom/gsupclient/gsup_peer_id.h>
+#include <osmocom/mslookup/mslookup.h>
+
 struct osmo_mslookup_query;
 struct osmo_mslookup_result;