media: vsp1: Export missing vsp1_isp_free_buffer symbol

The vsp1_isp_free_buffer() function implemented by the vsp1 driver is
part of the API exposed to the rcar-isp driver. All other symbols except
that one are properly exported. Fix it.

Fixes: d06c1a9f ("media: vsp1: Add VSPX support")
Cc: stable@vger.kernel.org
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Reviewed-by: Jacopo Mondi <jacopo.mondi@ideasonboard.com>
Reviewed-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Signed-off-by: Hans Verkuil <hverkuil+cisco@kernel.org>