comctl32: hotkey control needs to invalidate itself when painting for some apps

[Dan Kegel]

In at least one app, the hotkey control doesn't draw itself.
Invalidating its own window during HOTKEY_Paint gets the
control to draw properly, and makes the app happy.

Fixes http://bugs.winehq.org/show_bug.cgi?id=30486
(Unlike the patch attached to that bug, this one doesn't
also ask the background to be redrawn again.)