V4L/DVB (8178): uvc: Fix compilation breakage for the other drivers, if uvc is selected

UVC makefile defines obj as:
	obj-$(CONFIG_USB_VIDEO_CLASS) := uvcvideo.o
Instead of:
	obj-$(CONFIG_USB_VIDEO_CLASS) += uvcvideo.o

Due to that, if uvc is selected, all obj-y or obj-m that were added to
compilation were forget. This breaks a proper kernel build.
Acked-by: Laurent Pinchart <laurent.pinchart@skynet.be>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>

drivers/media/video/uvc/Makefile

 uvcvideo-objs  := uvc_driver.o uvc_queue.o uvc_v4l2.o uvc_video.o uvc_ctrl.o \
 		  uvc_status.o uvc_isight.o
-obj-$(CONFIG_USB_VIDEO_CLASS) := uvcvideo.o
+obj-$(CONFIG_USB_VIDEO_CLASS) += uvcvideo.o