Use proper type for tch_rtp_connect() parameter

Change-Id: I6e2efcd2e25d6ec2ff35a4b8cfcda02abe97fa59
diff --git a/src/libmsc/gsm_04_08_cc.c b/src/libmsc/gsm_04_08_cc.c
index 15c6d9d..5500f6f 100644
--- a/src/libmsc/gsm_04_08_cc.c
+++ b/src/libmsc/gsm_04_08_cc.c
@@ -1761,10 +1761,9 @@
 	return 0;
 }
 
-static int tch_rtp_connect(struct gsm_network *net, void *arg)
+static int tch_rtp_connect(struct gsm_network *net, struct gsm_mncc_rtp *rtp)
 {
 	struct gsm_trans *trans;
-	struct gsm_mncc_rtp *rtp = arg;
 	struct in_addr addr;
 
 	/* FIXME: in *rtp we should get the codec information of the remote