media: videodev2: Fix v4l2_ext_control packing.

The structure is packed, which requires that all its fields need to be
also packed.

./include/uapi/linux/videodev2.h:1810:2: warning: field  within 'struct v4l2_ext_control' is less aligned than 'union v4l2_ext_control::(anonymous at ./include/uapi/linux/videodev2.h:1810:2)' and is usually due to 'struct v4l2_ext_control' being packed, which can lead to unaligned accesses [-Wunaligned-access]

Explicitly set the inner union as packed.

Marking the inner union as 'packed' does not change the layout, since the
whole struct is already packed, it just silences the clang warning. See
also this llvm discussion:

https://github.com/llvm/llvm-project/issues/55520

Signed-off-by: Ricardo Ribalda <ribalda@chromium.org>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>