add json_object_object_length
diff --git a/linkhash.h b/linkhash.h
index bbb5488..378de0b 100644
--- a/linkhash.h
+++ b/linkhash.h
@@ -280,6 +280,7 @@
  */
 extern int lh_table_delete(struct lh_table *t, const void *k);
 
+extern int lh_table_length(struct lh_table *t);
 
 void lh_abort(const char *msg, ...);
 void lh_table_resize(struct lh_table *t, int new_size);