nat: Keep a ussd token in the config

We will have the USSD provider connecting to us and we
will use the IPA protocol, including the auth mechanism.
diff --git a/openbsc/include/openbsc/bsc_nat.h b/openbsc/include/openbsc/bsc_nat.h
index 61f19f2..f718766 100644
--- a/openbsc/include/openbsc/bsc_nat.h
+++ b/openbsc/include/openbsc/bsc_nat.h
@@ -235,6 +235,8 @@
 	/* USSD messages  we want to match */
 	char *ussd_lst_name;
 	char *ussd_query;
+	char *ussd_token;
+	char *ussd_local;
 
 	/* statistics */
 	struct bsc_nat_statistics stats;