iio: atlas-ph-sensor: Drop GPIO include

The driver includes <linux/gpio.h> yet fails to use symbols
from any the header so drop the include.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Matt Ranostay <matt.ranostay@konsulko.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>