extcon: max3355: Drop unused include

This driver includes the legacy <linux/gpio.h> header but
does not use it. Drop this include.

Cc: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>