compatibility with libknot's master (3.4 WIP)

I'm adding this as a function, as in knot-resolver 6.x we have
one more place where it is used, and I find this more readable.

lib/layer/validate.c

@@ -676,7 +676,7 @@ static int rrsig_not_found(const kr_layer_t * const ctx, const knot_pkt_t * cons
 	}
 	/* Add one extra label to cur_top, i.e. descend one level below current zone cut */
 	const knot_dname_t * const cut_next = rr->owner +
-		knot_dname_prefixlen(rr->owner, next_depth - 1, NULL);
+		kr_dname_prefixlen(rr->owner, next_depth - 1);
 
 	/* Spawn that DS sub-query. */
 	struct kr_query * const next = kr_rplan_push(&req->rplan, qry, cut_next,

lib/utils.h

@@ -606,3 +606,14 @@ KR_EXPORT long long kr_fssize(const char *path);
 /** Simply return de->dname. (useful from Lua) */
 KR_EXPORT const char * kr_dirent_name(const struct dirent *de);
 
+
+/* trivial libknot versions compatibility */
+static inline size_t kr_dname_prefixlen(const uint8_t *name, unsigned nlabels)
+{
+	return knot_dname_prefixlen(name, nlabels
+		#if KNOT_VERSION_HEX < 0x030400
+			, NULL
+		#endif
+	);
+}
+