iio: adc: select buffer for at91-sama5d2_adc

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>

drivers/iio/adc/Kconfig

@@ -159,6 +159,7 @@ config AT91_SAMA5D2_ADC
 	depends on ARCH_AT91 || COMPILE_TEST
 	depends on HAS_IOMEM
 	depends on HAS_DMA
+	select IIO_BUFFER
 	select IIO_TRIGGERED_BUFFER
 	help
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