gpio: forward-declare enum gpiod_flags

This enum is used in the gpiolib.h header file, yet
<linux/gpio/consumer.h> is not included so plainly including this
file (and some drivers do) will raise compile problems.
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

drivers/gpio/gpiolib.h

@@ -16,7 +16,7 @@
 #include <linux/device.h>
 
 enum of_gpio_flags;
-
+enum gpiod_flags;
 struct acpi_device;
 
 /**