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>