iio: Make possible to include driver.h first

If we put headers alphabetically sorted in the IIO driver,
the compilation will abort because of unknown type to handle.

Simple add a forward declaration of opaque struct iio_dev.

Suggested-by: Jonathan Cameron <jic23@kernel.org>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>