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>

drivers/gpio/gpiolib.h

@@ -14,6 +14,7 @@
 #include <linux/err.h>
 #include <linux/device.h>
 #include <linux/module.h>
+#include <linux/notifier.h>
 #include <linux/cdev.h>
 #include <linux/rwsem.h>