media: adv7511: Drop unused include

The adv7511 driver includes the legacy GPIO header <linux/gpio.h>
yet fails to use any symbols from it.

Drop the include.

Cc: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>