iio: adc: select buffer for at91-sama5d2_adc

commit 76974ef9 upstream.

We need to select the buffer code, otherwise we get build errors
with undefined functions on the trigger and buffer,
if we select just IIO and then AT91_SAMA5D2_ADC from menuconfig

This adds a Kconfig 'select' statement like other ADC
drivers have it already.

Fixes: 5e1a1da0 ("iio: adc: at91-sama5d2_adc: add hw trigger and buffer support")
Signed-off-by: Eugen Hristev <eugen.hristev@microchip.com>
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/iio/adc/Kconfig

@@ -158,6 +158,7 @@ config AT91_SAMA5D2_ADC
 	tristate "Atmel AT91 SAMA5D2 ADC"
 	depends on ARCH_AT91 || COMPILE_TEST
 	depends on HAS_IOMEM
+	select IIO_BUFFER
 	select IIO_TRIGGERED_BUFFER
 	help
 	  Say yes here to build support for Atmel SAMA5D2 ADC which is