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>

drivers/staging/iio/Documentation/iio_event_monitor.c

@@ -28,6 +28,7 @@
 #include <sys/ioctl.h>
 #include "iio_utils.h"
 #include <linux/iio/events.h>
+#include <linux/iio/types.h>
 
 static const char * const iio_chan_type_name_spec[] = {
 	[IIO_VOLTAGE] = "voltage",