backlight: lms501kf03: Drop unused include

This driver includes <linux/gpio.h> but does not use any
symbols from that file, 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>