[map] Another try at fixing old Mac build

https://github.com/harfbuzz/harfbuzz/issues/4138

src/hb-map.hh

@@ -343,7 +343,8 @@ struct hb_hashmap_t
   )
   auto keys () const HB_AUTO_RETURN
   (
-    + keys_ref ()
+    + iter_items ()
+    | hb_map (&item_t::key)
     | hb_map (hb_ridentity)
   )
   auto values_ref () const HB_AUTO_RETURN
@@ -353,7 +354,8 @@ struct hb_hashmap_t
   )
   auto values () const HB_AUTO_RETURN
   (
-    + values_ref ()
+    + iter_items ()
+    | hb_map (&item_t::value)
     | hb_map (hb_ridentity)
   )