[media] ov2640: make GPIOLIB an optional dependency

As warned by kbuild test robot:
	warning: (VIDEO_EM28XX_V4L2) selects VIDEO_OV2640 which has unmet direct dependencies (MEDIA_SUPPORT && VIDEO_V4L2 && I2C && GPIOLIB && MEDIA_CAMERA_SUPPORT)

The em28xx driver can use ov2640, but it doesn't depend
(or use) the GPIOLIB in order to power off/on the sensor.

So, as we want to allow both usages with and without
GPIOLIB, make its dependency optional.
Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>

drivers/media/i2c/Kconfig

@@ -519,7 +519,7 @@ config VIDEO_SMIAPP_PLL
 
 config VIDEO_OV2640
 	tristate "OmniVision OV2640 sensor support"
-	depends on VIDEO_V4L2 && I2C && GPIOLIB
+	depends on VIDEO_V4L2 && I2C
 	depends on MEDIA_CAMERA_SUPPORT
 	help
 	  This is a Video4Linux2 sensor-level driver for the OmniVision

drivers/media/i2c/ov2640.c

@@ -743,13 +743,16 @@ static int ov2640_s_power(struct v4l2_subdev *sd, int on)
 	struct i2c_client *client = v4l2_get_subdevdata(sd);
 	struct ov2640_priv *priv = to_ov2640(client);
 
-	gpiod_direction_output(priv->pwdn_gpio, !on);
+#ifdef CONFIG_GPIOLIB
+	if (priv->pwdn_gpio)
+		gpiod_direction_output(priv->pwdn_gpio, !on);
 	if (on && priv->resetb_gpio) {
 		/* Active the resetb pin to perform a reset pulse */
 		gpiod_direction_output(priv->resetb_gpio, 1);
 		usleep_range(3000, 5000);
 		gpiod_direction_output(priv->resetb_gpio, 0);
 	}
+#endif
 	return 0;
 }