leds: lm355x: Drop surplus include

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

Cc: G.Shark Jeong <gshark.jeong@gmail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Pavel Machek <pavel@ucw.cz>