media: videobuf2: core: Drop unneeded forward declaration

The static __enqueue_in_driver() function is forward-declared
needlessly, as it isn't called before its definition. Drop the forward
declaration.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Reviewed-by: Tomasz Figa <tfiga@chromium.org>