gpio: sysfs: fix forward declaration of struct gpio_device

The forward declaration for struct gpio_device should be provided for
both branches of the #ifdef.

Fixes: 08a149c4 ("gpiolib: Clean up headers")
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>