[media] media-entity: include linux/bug.h for WARN_ON

WARN_ON is used by this header file, but none of its direct includes
include asm/bug.h by way of linux/bug.h yet.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>