staging: iio: Documentation: iio_event_monitor: Include linux/iio/types.h

By adding this line and installing the kernel headers with
make headers_install, iio_event_monitor can be compiled without
any hacks.

Signed-off-by: Roberta Dobrescu <roberta.dobrescu@gmail.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>