cx25821: remove unused cx25821_video_wakeup() declaration

cx25821_video_wakeup() has been removed since
commit b671ae6b ("[media] cx25821: convert to vb2").

So cx25821_video_wakeup is a unused declaration, remove it.

Signed-off-by: Gaosheng Cui <cuigaosheng1@huawei.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>