USB: gadget webcam: depends on VIDEO_DEV

g_webcam uses v4l[2] interfaces, so it should depend on VIDEO_DEV.

ERROR: "v4l2_event_unsubscribe" [drivers/usb/gadget/g_webcam.ko] undefined!
ERROR: "v4l2_event_queue" [drivers/usb/gadget/g_webcam.ko] undefined!
ERROR: "video_device_release" [drivers/usb/gadget/g_webcam.ko] undefined!
ERROR: "video_usercopy" [drivers/usb/gadget/g_webcam.ko] undefined!
ERROR: "v4l2_event_dequeue" [drivers/usb/gadget/g_webcam.ko] undefined!
ERROR: "video_register_device" [drivers/usb/gadget/g_webcam.ko] undefined!
ERROR: "video_device_alloc" [drivers/usb/gadget/g_webcam.ko] undefined!
ERROR: "v4l2_event_subscribe" [drivers/usb/gadget/g_webcam.ko] undefined!
ERROR: "video_unregister_device" [drivers/usb/gadget/g_webcam.ko] ndefined!
ERROR: "v4l2_event_pending" [drivers/usb/gadget/g_webcam.ko] undefined!
ERROR: "v4l2_fh_init" [drivers/usb/gadget/g_webcam.ko] undefined!
ERROR: "v4l2_event_init" [drivers/usb/gadget/g_webcam.ko] undefined!
ERROR: "video_devdata" [drivers/usb/gadget/g_webcam.ko] undefined!
ERROR: "v4l2_event_alloc" [drivers/usb/gadget/g_webcam.ko] undefined!
ERROR: "v4l2_fh_add" [drivers/usb/gadget/g_webcam.ko] undefined!
ERROR: "v4l2_fh_del" [drivers/usb/gadget/g_webcam.ko] undefined!
ERROR: "v4l2_fh_exit" [drivers/usb/gadget/g_webcam.ko] undefined!

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>