media: v4l2-compat-ioctl32: Copy v4l2_window->global_alpha

Commit b2787845 ("V4L/DVB (5289): Add support for video output
overlays.") added the field global_alpha to struct v4l2_window but did
not update the compat layer accordingly. This change adds global_alpha
to struct v4l2_window32 and copies the value for global_alpha back and
forth.

Signed-off-by: Daniel Mentz <danielmentz@google.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>