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>