iio: declare struct to fix warning
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: Pengyu Ma <25ead0e03840c5784199a8a76ba92adb3ac382cf@windriver.com>
Acked-by: Daniel Baluta <f45a6396254d0683f1df8e14b691f61d753fe117@intel.com>
Signed-off-by: Jonathan Cameron <09f65b71b7655725897b2fd41a09a0cefe2e1ace@kernel.org>

KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs