drm/rockchip: fb: add missing header

This fixes the following sparse warnings:

drivers/gpu/drm/rockchip/rockchip_drm_fb.c:32:23: warning: symbol 'rockchip_fb_get_gem_obj' was not declared. Should it be static?
drivers/gpu/drm/rockchip/rockchip_drm_fb.c:315:24: warning: symbol 'rockchip_drm_framebuffer_init' was not declared. Should it be static?
drivers/gpu/drm/rockchip/rockchip_drm_fb.c:329:6: warning: symbol 'rockchip_drm_mode_config_init' was not declared. Should it be static?
Signed-off-by: NJohn Keeping <john@metanate.com>

drivers/gpu/drm/rockchip/rockchip_drm_fb.c

@@ -20,6 +20,7 @@
 #include <drm/drm_crtc_helper.h>
 
 #include "rockchip_drm_drv.h"
+#include "rockchip_drm_fb.h"
 #include "rockchip_drm_gem.h"
 
 #define to_rockchip_fb(x) container_of(x, struct rockchip_drm_fb, fb)