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: NHulk Robot <hulkci@huawei.com>
Signed-off-by: NYueHaibing <yuehaibing@huawei.com>
Acked-by: NMaxime Jourdan <mjourdan@baylibre.com>
Signed-off-by: NHans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: NMauro Carvalho Chehab <mchehab+samsung@kernel.org>

drivers/staging/media/meson/vdec/vdec.c

@@ -12,6 +12,7 @@
 #include <linux/mfd/syscon.h>
 #include <linux/slab.h>
 #include <linux/interrupt.h>
+#include <linux/kthread.h>
 #include <media/v4l2-ioctl.h>
 #include <media/v4l2-event.h>
 #include <media/v4l2-ctrls.h>