backlight: ams369fg06: 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: Daniel Thompson <daniel.thompson@linaro.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>