iio: adc: dln2-adc: fix build error

The dln2-adc driver uses interface(s) that are controlled by the
IIO_TRIGGERED_BUFFER Kconfig symbol, so the driver needs to select
that symbol to prevent the build error.

drivers/iio/adc/dln2-adc.o: In function `dln2_adc_probe':
dln2-adc.c:(.text+0x528): undefined reference to `devm_iio_triggered_buffer_setup'

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Reported-by: kbuild test robot <fengguang.wu@intel.com>
Cc: Jonathan Cameron <jic23@kernel.org>
Cc: linux-iio@vger.kernel.org
Cc: Jack Andersen <jackoalan@gmail.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>