iio: adc: ad7944: add missing select IIO_(TRIGGERED_)BUFFER in Kconfig

iio: adc: ad7944: add missing select IIO_(TRIGGERED_)BUFFER in Kconfig

This driver makes use of triggered buffers, but does not select the
required modules.

Add the missing 'select IIO_BUFFER' and 'select IIO_TRIGGERED_BUFFER'.

Fixes: d1efcf8871db ("iio: adc: ad7944: add driver for AD7944/AD7985/AD7986")
Signed-off-by: Javier Carrasco <javier.carrasco.cruz@gmail.com>
Reviewed-by: David Lechner <dlechner@baylibre.com>
Link: https://patch.msgid.link/20241003-iio-select-v1-2-67c0385197cd@gmail.com
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>