[media] videodev2: Add V4L2_BUF_FLAG_LAST

This v4l2_buffer flag can be used by drivers to mark a capture buffer
as the last generated buffer, for example after a V4L2_DEC_CMD_STOP
command was issued.

Signed-off-by: Peter Seiderer <ps.report@gmx.net>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Acked-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Kamil Debski <k.debski@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>