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>

drivers/iio/adc/hi8435.c

@@ -17,7 +17,6 @@
 #include <linux/module.h>
 #include <linux/of.h>
 #include <linux/of_device.h>
-#include <linux/of_gpio.h>
 #include <linux/spi/spi.h>
 #include <linux/gpio/consumer.h>