fbdev: Do not include <linux/backlight.h> in header

Forward declare struct backlight_device and remove the include
statement.
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Acked-by: Helge Deller <deller@gmx.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20240219093941.3684-5-tzimmermann@suse.de

include/linux/fb.h

@@ -13,11 +13,11 @@
 #include <linux/workqueue.h>
 #include <linux/notifier.h>
 #include <linux/list.h>
-#include <linux/backlight.h>
 #include <linux/slab.h>
 
 #include <asm/fb.h>
 
+struct backlight_device;
 struct vm_area_struct;
 struct fb_info;
 struct device;