iio: srf08: fix link error "devm_iio_triggered_buffer_setup" undefined

Functions for triggered buffer support are needed by this module.
If they are not defined accidentally by another driver, there's an error
thrown out while linking.

Add a select of IIO_BUFFER and IIO_TRIGGERED_BUFFER in the Kconfig file.

Signed-off-by: Andreas Klinger <ak@it-klinger.de>
Fixes: a8319593 ("iio: srf08: add triggered buffer support")
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>