iio:adc:hi8435: Drop unused of_gpio.h header

I guess this is a left over from some earlier cleanup. Not used
currently.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Cc: Nikita Yushchenko <nikita.yoush@cogentembedded.com>
Cc: Vladimir Barinov <vladimir.barinov@cogentembedded.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>