staging: iio: select IRQ_WORK for IIO_DUMMY_EVGEN

The iio dummy code was recently changed to use irq_work_queue, but
that code is compiled into the kernel only if IRQ_WORK is set, so
we can get a link error here:

drivers/built-in.o: In function `iio_evgen_poke':
(.text+0x208a04): undefined reference to `irq_work_queue'

This changes the Kconfig file to match what other drivers do.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Fixes: fd2bb310 ("Staging: iio: Move evgen interrupt generation to irq_work")
Acked-by: Daniel Baluta <daniel.baluta@intel.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>