media: meson: vdec: Add missing kthread.h

Fix building error:

 of function kthread_should_stop;
 did you mean thread_saved_sp? [-Werror=implicit-function-declaration]

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Acked-by: Maxime Jourdan <mjourdan@baylibre.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>