MNCC: clarify naming / rename mncc_recv() to int_mncc_recv()

the int_ perfix (internal) symbolizes that this is the internal
MNCC handler code, receiving messages from the MNCC interface point.
diff --git a/openbsc/src/mncc.c b/openbsc/src/mncc.c
index 180b626..328e96c 100644
--- a/openbsc/src/mncc.c
+++ b/openbsc/src/mncc.c
@@ -347,7 +347,8 @@
 }
 
 
-int mncc_recv(struct gsm_network *net, int msg_type, void *arg)
+/* Internal MNCC handler input function (from CC -> MNCC -> here) */
+int int_mncc_recv(struct gsm_network *net, int msg_type, void *arg)
 {
 	struct gsm_mncc *data = arg;
 	int callref;