winemac.drv: Make sure that NSMiniaturizableWindowMask style mask is set before calling miniaturize.

miniaturize fails to minimize window when NSMiniaturizableWindowMask
style is not set. The style will be restored on window restore (or earlier).
Signed-off-by: Piotr Caban <piotr@codeweavers.com>
Signed-off-by: Ken Thomases <ken@codeweavers.com>
Signed-off-by: Alexandre Julliard <julliard@winehq.org>