nat: Add statistics to the access-list in the NAT

Count how many times we match a BSC or NAT deny. This will
give us the number of how often something should be filtered.
diff --git a/openbsc/include/openbsc/bsc_nat.h b/openbsc/include/openbsc/bsc_nat.h
index ff0ee48..f988a8f 100644
--- a/openbsc/include/openbsc/bsc_nat.h
+++ b/openbsc/include/openbsc/bsc_nat.h
@@ -152,9 +152,17 @@
 	} msc;
 };
 
+enum bsc_nat_acc_ctr {
+	ACC_LIST_BSC_FILTER,
+	ACC_LIST_NAT_FILTER,
+};
+
 struct bsc_nat_acc_lst {
 	struct llist_head list;
 
+	/* counter */
+	struct rate_ctr_group *stats;
+
 	/* the name of the list */
 	const char *name;
 	struct llist_head fltr_list;