iio:ti_am335x_adc ensure that IIO_KFIFO_BUF is not selected witout IIO_BUFFER

This avoids build problems such iio_push_to_buffers* not being defined.
Signed-off-by: NJonathan Cameron <jic23@kernel.org>
Reported-by: Nkbuild test robot <fengguang.wu@intel.com>

drivers/iio/adc/Kconfig

@@ -177,6 +177,7 @@ config TI_ADC081C
 config TI_AM335X_ADC
 	tristate "TI's AM335X ADC driver"
 	depends on MFD_TI_AM335X_TSCADC
+	select IIO_BUFFER
 	select IIO_KFIFO_BUF
 	help
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