Add @:privateAccess metadata to Promise.when
Add @:privateAccess metadata to Promise.when

Since https://github.com/HaxeFoundation/haxe/commit/a74040ecc0b567929dae231a5f0d20761baeb44a this is needed.
Fixes https://github.com/jdonaldson/promhx/issues/51
Blank101/promhx,massiveinteractive/promhx,francescoagati/promhx