gpiolib: add gpio_device_get_label() stub for !GPIOLIB

Add empty stub of gpio_device_get_label() when GPIOLIB is not enabled.

Cc: <stable@vger.kernel.org>
Fixes: d1f77282 ("gpiolib: provide gpio_device_get_label()")
Suggested-by: kernel test robot <lkp@intel.com>
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>

include/linux/gpio/driver.h

@@ -831,6 +831,12 @@ static inline int gpio_device_get_base(struct gpio_device *gdev)
 	return -ENODEV;
 }
 
+static inline const char *gpio_device_get_label(struct gpio_device *gdev)
+{
+	WARN_ON(1);
+	return NULL;
+}
+
 static inline int gpiochip_lock_as_irq(struct gpio_chip *gc,
 				       unsigned int offset)
 {