media: ov7670: Drop unused include

The driver includes the legacy <linux/gpio.h> header but does
not use any symbols from it. Drop the include.

Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Akinobu Mita <akinobu.mita@gmail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>