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 <arnd@arndb.de>
Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Daniel Baluta <daniel.baluta@intel.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>