drm: rcar-du: of: Include header to define prototypes

drm: rcar-du: of: Include header to define prototypes

The symbol 'rcar_du_of_init' is defined by the rcar_du_of module header,
but it is not included by the C implementation.

Include the header to correctly define the function prototypes.

Fixes the following warning:

linux/drivers/gpu/drm/rcar-du/rcar_du_of.c:319:13:
   warning: symbol 'rcar_du_of_init' was not declared. Should it be static?
    CC      drivers/gpu/drm/rcar-du/rcar_du_of.o

Fixes: 81c0e3dd8292 ("drm: rcar-du: Fix legacy DT to create LVDS encoder nodes")
Signed-off-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>
Reviewed-by: Vaishali Thakkar <vthakkar@vaishalithakkar.in>
Reviewed-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>