usb: gadget: Make webcam gadget select USB_LIBCOMPOSITE

Composite gadget support is now available as a library instead of being
built with each gadget. Composite drivers need to select
USB_LIBCOMPOSITE.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>