V4L/DVB (5402): Add vsync_field to the union in video_event for VIDEO_EVENT_VSYNC

VIDEO_EVENT_VSYNC needs to tell the application which field it was that
received a VSYNC (odd/even/progressive). The vsync_field was added to the
union in video_event for this purpose.

Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>