backlight: Include <linux/of.h>

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

Signed-off-by: Thomas Zimmermann <[e-mail]>
Reviewed-by: "Daniel Thompson (RISCstar)" <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Lee Jones <[e-mail]>

drivers/video/backlight/backlight.c

···
 #include <linux/ctype.h>
 #include <linux/err.h>
 #include <linux/slab.h>
+#include <linux/of.h>
 
 #ifdef CONFIG_PMAC_BACKLIGHT
 #include <asm/backlight.h>