media: v4l2-dev.h: move open brace after struct video_device

The open brace should be after a struct of define, that's good
for searching.

Signed-off-by: Yajun Deng <yajun.deng@linux.dev>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>