videodev2.h: add p_s32 and p_s64 pointers

Added p_s32 and p_s64 pointers to the union in struct v4l2_ext_control to
simplify INTEGER and INTEGER64 control array support.

Internally the control framework handles such arrays just fine, but
userspace is missing corresponding pointers to access array elements of
these types.

The internal union v4l2_ctrl_ptr which the control framework uses already
has these types, they just were never added to the public API.

Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>