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: Nkbuild test robot <fengguang.wu@intel.com>
Signed-off-by: NDaniel Baluta <daniel.baluta@intel.com>
Signed-off-by: NJonathan Cameron <jic23@kernel.org>

drivers/staging/iio/Kconfig

@@ -38,6 +38,7 @@ config IIO_SIMPLE_DUMMY_EVENTS
 config IIO_SIMPLE_DUMMY_BUFFER
 	bool "Buffered capture support"
 	select IIO_BUFFER
+	select IIO_TRIGGER
 	select IIO_KFIFO_BUF
 	help
 	  Add buffered data capture to the simple dummy driver.