iio: Drop duplicate forward declaration

Commit 5f420b42079c ("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 7ae8cf627558 ("staging: iio: chrdev.h rationalization").

Cc: Lars-Peter Clausen <[e-mail]>
Signed-off-by: Lukas Wunner <[e-mail]>
Signed-off-by: Jonathan Cameron <[e-mail]>

include/linux/iio/iio.h

···
 #define INDIO_MAX_RAW_ELEMENTS		4
 
 struct iio_trigger; /* forward declaration */
-struct iio_dev;
 
 /**
  * struct iio_info - constant information about device