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>

include/linux/iio/driver.h

@@ -11,6 +11,7 @@
 #ifndef _IIO_INKERN_H_
 #define _IIO_INKERN_H_
 
+struct iio_dev;
 struct iio_map;
 
 /**