leds: lt3593: Drop surplus include

This driver is already including and using the proper
<linux/gpio/consumer.h> header, there is no need to include
the legacy <linux/gpio.h> header.

Cc: Daniel Mack <daniel@zonque.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Pavel Machek <pavel@ucw.cz>