iio: adc: stm32-dfsdm: fix triggered buffer build dependency

This fixes build errors seen when CONFIG_STM32_DFSDM_ADC is set, as
stm32-dfsdm-adc driver now also relies on triggered buffer API:

Fixes: 11646e81 ("iio: adc: stm32-dfsdm: add support for buffer modes")
Signed-off-by: Fabrice Gasnier <fabrice.gasnier@st.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/adc/Kconfig

@@ -823,6 +823,7 @@ config STM32_DFSDM_ADC
 	select REGMAP_MMIO
 	select IIO_BUFFER
 	select IIO_BUFFER_HW_CONSUMER
+	select IIO_TRIGGERED_BUFFER
 	help
 	  Select this option to support ADCSigma delta modulator for
 	  STMicroelectronics STM32 digital filter for sigma delta converter.