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>