staging: iio: dummy: Fix undefined symbol build error
staging: iio: dummy: Fix undefined symbol build error

CONFIG_SIMPLE_DUMMY_BUFFER compiles in iio_simple_dummy_buffer.c file
which uses functions from industrialio-trigger.c.

So, CONFIG_SIMPLE_DUMMY_BUFFER needs to select IIO_TRIGGER in order to
avoid build error like this:

> ERROR: "iio_trigger_notify_done" [drivers/staging/iio/iio_dummy.ko] undefined!
> ERROR: "iio_triggered_buffer_postenable" [drivers/staging/iio/iio_dummy.ko] undefined!
> ERROR: "iio_triggered_buffer_predisable" [drivers/staging/iio/iio_dummy.ko] undefined!
> ERROR: "iio_alloc_pollfunc" [drivers/staging/iio/iio_dummy.ko] undefined!
> ERROR: "iio_dealloc_pollfunc" [drivers/staging/iio/iio_dummy.ko] undefined!

Cc: Arnd Bergmann <f2c659f01951776204a6c5b902787d9019fbeebd@arndb.de>
Reported-by: kbuild test robot <24f7fe9d205c8a9f6ade0c2894e14303ca16087f@intel.com>
Signed-off-by: Daniel Baluta <f45a6396254d0683f1df8e14b691f61d753fe117@intel.com>
Signed-off-by: Jonathan Cameron <09f65b71b7655725897b2fd41a09a0cefe2e1ace@kernel.org>

KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs