firmware: sysfb: Don't use "proxy" headers

Update header inclusions to follow IWYU (Include What You Use)
principle.

Note that kernel.h is discouraged to be included as it's written
at the top of that file.

Reviewed-by: Javier Martinez Canillas <[e-mail]>
Reviewed-by: Thomas Zimmermann <[e-mail]>
Signed-off-by: Andy Shevchenko <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Javier Martinez Canillas <[e-mail]>

include/linux/sysfb.h

···
  * Copyright (c) 2012-2013 David Herrmann <[e-mail]>
  */
 
-#include <linux/kernel.h>
+#include <linux/err.h>
+#include <linux/types.h>
+
 #include <linux/platform_data/simplefb.h>
 
+struct device;
+struct platform_device;
 struct screen_info;
 
 enum {