msc: Add msc ip-tos NR option for the BSC

Allow to set the TOS field via the VTY interface. The
SO_PRIORITY was not used as it has no effect on the
packets being sent (in contrast to the documentation).
diff --git a/openbsc/src/nat/bsc_nat.c b/openbsc/src/nat/bsc_nat.c
index c80c088..3b3bf69 100644
--- a/openbsc/src/nat/bsc_nat.c
+++ b/openbsc/src/nat/bsc_nat.c
@@ -1110,7 +1110,7 @@
 		return -4;
 
 	/* connect to the MSC */
-	nat->msc_con = bsc_msc_create(nat->msc_ip, nat->msc_port);
+	nat->msc_con = bsc_msc_create(nat->msc_ip, nat->msc_port, 0);
 	if (!nat->msc_con) {
 		fprintf(stderr, "Creating a bsc_msc_connection failed.\n");
 		exit(1);