iio: dummy: Fix build error when CONFIG_IIO_TRIGGERED_BUFFER is not set

Gcc reports build error when CONFIG_IIO_TRIGGERED_BUFFER is not set:

riscv64-linux-gnu-ld: drivers/iio/dummy/iio_simple_dummy_buffer.o: in function `iio_simple_dummy_configure_buffer':
iio_simple_dummy_buffer.c:(.text+0x2b0): undefined reference to `iio_triggered_buffer_setup_ext'
riscv64-linux-gnu-ld: drivers/iio/dummy/iio_simple_dummy_buffer.o: in function `.L0 ':
iio_simple_dummy_buffer.c:(.text+0x2fc): undefined reference to `iio_triggered_buffer_cleanup'

Fix it by select IIO_TRIGGERED_BUFFER for config IIO_SIMPLE_DUMMY_BUFFER.

Fixes: 738f6ba1

 ("iio: dummy: iio_simple_dummy_buffer: use triggered buffer core calls")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>