dt-bindings: iio: dac: adi,ad5755: drop unrelated included.

Probably a cut and paste error, but the binding header used in the
example is for the wrong device and nothing from it is used.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Cc: Sean Nyekjaer <sean.nyekjaer@prevas.dk>
Reviewed-by: Rob Herring <robh@kernel.org>