nat: Store the IMSI inside the SCCP Connection data

Store the IMSI for the connections that we are tracking,
it will be freed when the normnal SCCP connection is freed.
diff --git a/openbsc/src/nat/bsc_nat_utils.c b/openbsc/src/nat/bsc_nat_utils.c
index c1e3c98..9e4ccc6 100644
--- a/openbsc/src/nat/bsc_nat_utils.c
+++ b/openbsc/src/nat/bsc_nat_utils.c
@@ -469,6 +469,7 @@
 
 	ret = auth_imsi(bsc, mi_string);
 	con->imsi_checked = 1;
+	con->imsi = talloc_strdup(con, mi_string);
 	return ret;
 }