staging:iio:dummy fix kfifo_buf kconfig dependency issue if kfifo modular and buffer...

This only occurs in the unlikely event that the example driver is built
in whilst the buffer implementation is not.

Solved by switching from a depends on to a select for this particular case.

Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Reported-by: Randy Dunlap <rdunlap@infradead.org>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>