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

drivers/iio/dummy/Kconfig

@@ -5,7 +5,8 @@ menu "IIO dummy driver"
 	depends on IIO
 
 config IIO_DUMMY_EVGEN
-       tristate
+	select IRQ_WORK
+	tristate
 
 config IIO_SIMPLE_DUMMY
        tristate "An example driver with no hardware requirements"