nat: Keep track of the MSC handling the connection.

In the future we might have multiple MSCs connected
and need to dispatch it to the right one.
diff --git a/openbsc/src/nat/bsc_nat.c b/openbsc/src/nat/bsc_nat.c
index 356025c..ad5fcd1 100644
--- a/openbsc/src/nat/bsc_nat.c
+++ b/openbsc/src/nat/bsc_nat.c
@@ -703,6 +703,7 @@
 			if (!create_sccp_src_ref(bsc, parsed))
 				goto exit2;
 			con = patch_sccp_src_ref_to_msc(msg, parsed, bsc);
+			con->msc_con = bsc->nat->msc_con;
 			con->con_type = con_type;
 			con_found = 1;
 			con_bsc = con->bsc;