[media] media: Include linux/kernel.h for DIV_ROUND_UP()

DIV_ROUND_UP() is defined in kernel.h which was not included by
media-entity.h. Do exactly that.

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>