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 <pengyu.ma@windriver.com>
Acked-by: Daniel Baluta <daniel.baluta@intel.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>