fbdev/chipsfb: Include <linux/backlight.h>

Fix builds with CONFIG_PMAC_BACKLIGHT=y. The include statement for
the backlight header has recently been removed from <linux/fb.h>.

Reported-by: Naresh Kamboju <naresh.kamboju@linaro.org>
Closes: https://lore.kernel.org/dri-devel/CA+G9fYsAk5TbqqxFC2W4oHLGA0CbTHMxbeq8QayFXTU75YiueA@mail.gmail.com/
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Fixes: 11b4eed ("fbdev: Do not include <linux/backlight.h> in header")
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Jani Nikula <jani.nikula@intel.com>
Cc: Helge Deller <deller@gmx.de>
Cc: linux-fbdev@vger.kernel.org
Cc: dri-devel@lists.freedesktop.org
Link: https://patchwork.freedesktop.org/patch/msgid/20240306122935.10626-2-tzimmermann@suse.de

Author: Thomas Zimmermann

drivers/video/fbdev/chipsfb.c

@@ -15,6 +15,7 @@
  */
 
 #include <linux/aperture.h>
+#include <linux/backlight.h>
 #include <linux/module.h>
 #include <linux/kernel.h>
 #include <linux/errno.h>