iio: adc: ti-adc081c: Select IIO_TRIGGERED_BUFFER to prevent build errors

Commit 08e05d1f ("ti-adc081c: Initial triggered buffer support") added
triggered buffer support but that also requires CONFIG_IIO_TRIGGERED_BUFFER,
otherwise we get errors from linker such as:

  drivers/built-in.o: In function `adc081c_remove':
  drivers/iio/adc/ti-adc081c.c:225: undefined reference to `iio_triggered_buffer_cleanup'

Fix these by explicitly selecting both CONFIG_IIO_TRIGGERED_BUFFER and
CONFIG_IIO_BUFFER in Kconfig for the driver.

Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>