gpiolib: add missing include

gpiolib.h uses notifiers but doesn't include <linux/notifier.h>.

Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>
Reviewed-by: Kent Gibson <warthog618@gmail.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>