iio: chemical: pms7003: fix unmet triggered buffer dependency

iio: chemical: pms7003: fix unmet triggered buffer dependency

IIO triggered buffer depends on IIO buffer which is missing from Kconfig
file. This should go unnoticed most of the time because there's a
chance something else has already enabled buffers. In some rare cases
though one might experience kbuild warnings about unmet direct
dependencies and build failures due to missing symbols.

Fix this by selecting IIO_BUFFER explicitly.

Signed-off-by: Tomasz Duszynski <tduszyns@gmail.com>
Fixes: 27c2023ab29b ("iio: chemical: add support for Plantower PMS7003 sensor")
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>