iio: pressure: ms5611: select IIO_BUFFER

The ms5611 driver started using the IIO_TRIGGERED_BUFFER infrastructure
which in turn depend on IIO_BUFFER, and it produces a build error now
if that is not enabled:

warning: (... && MS5611 && ...) selects IIO_TRIGGERED_BUFFER which has unmet direct dependencies (IIO && IIO_BUFFER)
buffer/industrialio-triggered-buffer.c: In function 'iio_triggered_buffer_setup':
buffer/industrialio-triggered-buffer.c:58:2: error: implicit declaration of function 'iio_device_attach_buffer' [-Werror=implicit-function-declaration]
pressure/ms5611_core.c: In function 'ms5611_trigger_handler':
pressure/ms5611_core.c:193:2: error: implicit declaration of function 'iio_push_to_buffers_with_timestamp' [-Werror=implicit-function-declaration]

This adds the second select.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Fixes: 713bbb4e ("iio: pressure: ms5611: Add triggered buffer support")
Acked-by: Daniel Baluta <daniel.baluta@intel.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>