backlight: Include <linux/of.h>

Include <linux/of.h> to avoid dependency on backlight header to
include it.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: "Daniel Thompson (RISCstar)" <danielt@kernel.org>
Link: https://lore.kernel.org/r/20250715122643.137027-7-tzimmermann@suse.de
Signed-off-by: Lee Jones <lee@kernel.org>

Author: tdz

drivers/video/backlight/backlight.c

@@ -16,6 +16,7 @@
 #include <linux/ctype.h>
 #include <linux/err.h>
 #include <linux/slab.h>
+#include <linux/of.h>
 
 #ifdef CONFIG_PMAC_BACKLIGHT
 #include <asm/backlight.h>