MNCC: introduce mncc_upq_enqueue() wrapper around msgb_enqueue()

This wrapper will allow us to add queue related code at one single
location in the code as opposed to three locations.
diff --git a/openbsc/src/gsm_04_08.c b/openbsc/src/gsm_04_08.c
index 521dab4..5eb6a24 100644
--- a/openbsc/src/gsm_04_08.c
+++ b/openbsc/src/gsm_04_08.c
@@ -1330,7 +1330,7 @@
 	if (!msg)
 		return -ENOMEM;
 	memcpy(msg->data, mncc, sizeof(struct gsm_mncc));
-	msgb_enqueue(&net->upqueue, msg);
+	mncc_upq_enqueue(net, msg);
 
 	return 0;
 }