[media] videodev2.h: Fix VIDIOC_QUERYMENU ioctl regression

Fixes a regression in VIDIOC_QUERYMENU introduced when the __s64 value
field was added to the union. On a 64-bit system this will change the
size of this v4l2_querymenu structure from 44 to 48 bytes, thus
breaking the ABI. By adding the packed attribute it is working again.

Tested on both 64 and 32 bit systems.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Acked-by: Sakari Ailus <sakari.ailus@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>