[Wayland] Modifiers not working in wheelEvent

To reproduce with the attached test application:

1. Run weston or kwin_wayland
2. Run the app with --platform wayland
3. Press one modifier key (Ctrl, Shift, Alt or Meta)
4. Scroll with the mouse wheel

In the wheelEvent() function, QWheelEvent::modifiers() doesn't report the modifiers being held, it just returns NoModifier.

This feature works as expected with --platform xcb.