gpiolib: add gpio_device_get_label() stub for !GPIOLIB

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

Cc: <[e-mail]>
Fixes: d1f7728259ef ("gpiolib: provide gpio_device_get_label()")
Suggested-by: kernel test robot <[e-mail]>
Signed-off-by: Krzysztof Kozlowski <[e-mail]>
Signed-off-by: Bartosz Golaszewski <[e-mail]>

include/linux/gpio/driver.h

···
 	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)
 {