iio: declare struct to fix warning

When compile iio related driver the following warning shown:

include/linux/iio/trigger.h:35:34: warning: 'struct iio_trigger'
declared inside parameter list
  int (*set_trigger_state)(struct iio_trigger *trig, bool state);

include/linux/iio/trigger.h:38:18: warning: 'struct iio_dev'
declared inside parameter list
           struct iio_dev *indio_dev);

'struct iio_dev' and 'struct iio_trigger' was used before declaration,
forward declaration for these structs to fix warning.
Signed-off-by: NPengyu Ma <pengyu.ma@windriver.com>
Acked-by: NDaniel Baluta <daniel.baluta@intel.com>
Signed-off-by: NJonathan Cameron <jic23@kernel.org>

include/linux/iio/trigger.h

@@ -18,6 +18,9 @@ struct iio_subirq {
 	bool enabled;
 };
 
+struct iio_dev;
+struct iio_trigger;
+
 /**
  * struct iio_trigger_ops - operations structure for an iio_trigger.
  * @owner:		used to monitor usage count of the trigger.