video: fbdev: omap: Add prototype for hwa742_update_window_async()

The symbol hwa742_update_window_async() is exported, but there is no
prototype defined for it. That's why gcc complains:

drivers-video-fbdev-omap-hwa742.c:warning:no-previous-prototype-for-hwa742_update_window_async

Add the prototype, but I wonder if we couldn't drop exporting the symbol
instead. Since omapfb_update_window_async() is exported the same way,
are there any users outside of the tree?

Signed-off-by: Helge Deller <deller@gmx.de>