iio: chemical: select IRQ_WORK for ATLAS ph sensor

The newly added atlas-ph-sensor driver uses irq_work_queue, which
may not always be enabled:

ERROR: "irq_work_queue" [drivers/iio/chemical/atlas-ph-sensor.ko] undefined!

This adds a 'select' statement to Kconfig to ensure it's there
when we need it.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Fixes: 27dec00e ("iio: chemical: add Atlas pH-SM sensor support")
Reviewed-by: Matt Ranostay <mranostay@gmail.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>