macintosh/via-pmu-backlight: 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>. Add it to
via-pmu-backlight.c to get the necessary symbols.

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/20240306122935.10626-3-tzimmermann@suse.de

Author: Thomas Zimmermann

drivers/macintosh/via-pmu-backlight.c

@@ -10,6 +10,7 @@
 
 #include <asm/ptrace.h>
 #include <linux/adb.h>
+#include <linux/backlight.h>
 #include <linux/pmu.h>
 #include <asm/backlight.h>