iio: Drop duplicate forward declaration

Commit 5f420b42 ("staging:iio: Add extended IIO channel info") added
a forward declaration for struct iio_dev to <linux/iio/iio.h> but forgot
to remove an existing forward declaration further down originating from
commit 7ae8cf62 ("staging: iio: chrdev.h rationalization").

Cc: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Lukas Wunner <lukas@wunner.de>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

include/linux/iio/iio.h

@@ -365,7 +365,6 @@ unsigned int iio_get_time_res(const struct iio_dev *indio_dev);
 #define INDIO_MAX_RAW_ELEMENTS		4
 
 struct iio_trigger; /* forward declaration */
-struct iio_dev;
 
 /**
  * struct iio_info - constant information about device