The new writeback feature is exports functions so that they can^^ not needed?
integrate into the rcar_du_kms module.[...]
The interface functions are defined in the rcar_du_writeback header, but
it is not included in the object file itself leading to compiler
warnings for missing prototypes.
Include the header as appropriate.
Signed-off-by: Kieran Bingham <kieran.bingham+renesas@xxxxxxxxxxxxxxxx>