osmux: Allow to listen to a specific address

For a setup with multiple network interfaces be able to pick
the one that osmux should be used/visible.
diff --git a/openbsc/src/libmgcp/mgcp_protocol.c b/openbsc/src/libmgcp/mgcp_protocol.c
index 42ce8bb..1819cea 100644
--- a/openbsc/src/libmgcp/mgcp_protocol.c
+++ b/openbsc/src/libmgcp/mgcp_protocol.c
@@ -1193,6 +1193,7 @@
 
 	cfg->source_port = 2427;
 	cfg->source_addr = talloc_strdup(cfg, "0.0.0.0");
+	cfg->osmux_addr = talloc_strdup(cfg, "0.0.0.0");
 
 	cfg->transcoder_remote_base = 4000;