iio: adc: ad4695: Add missing Kconfig select

Add select IIO_BUFFER and select IIO_TRIGGERED_BUFFER to the Kconfig for
the ad4695 driver.

Fixes: 6cc7e4bf ("iio: adc: ad4695: implement triggered buffer")
Signed-off-by: David Lechner <dlechner@baylibre.com>
Reviewed-by: Javier Carrasco <javier.carrasco.cruz@gmail.com>
Link: https://patch.msgid.link/20241009-iio-adc-ad4695-fix-kconfig-v1-1-e2a4dfde8d55@baylibre.com


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>