staging:iio:iio_utils.h: Add missing include

iio_utils.h uses opendir and friends which need dirent.h

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <jic23@cam.ac.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>