iio: core: iio_chan_spec_ext_info: Add private handle

There is currently no user, but we might need it in future.
So better add it now, before we have to convert drivers afterwards.

Signed-off-by: Michael Hennerich <michael.hennerich@analog.com>
Acked-by: Jonathan Cameron <jic23@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>