iio: adc: ad7779: add missing 'select IIO_TRIGGERED_BUFFER' to Kconfig

iio: adc: ad7779: add missing 'select IIO_TRIGGERED_BUFFER' to Kconfig

The Kconfig entry for the AD7779 is missing a
'select IIO_TRIGGERED_BUFFER' parameter, causing build failures.

Fixes: c9a3f8c7bfcb ("drivers: iio: adc: add support for ad777x family")
Cc: stable@vger.kernel.org
Signed-off-by: Joshua Crofts <joshua.crofts1@gmail.com>
Tested-by: Andy Shevchenko <andriy.shevchenko@intel.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@intel.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>