Sony-laptop: Fix sparse warning (make undeclared var static)

Fix the following sparse warning:

sony-laptop.c:1035:29: warning: symbol 'sony_bl_props' was not declared. Should it be static?
Signed-off-by: Lad, Prabhakar <prabhakar.csengg@gmail.com>
Signed-off-by: Darren Hart <dvhart@linux.intel.com>

drivers/platform/x86/sony-laptop.c

@@ -1032,7 +1032,7 @@ struct sony_backlight_props {
 	u8			offset;
 	u8			maxlvl;
 };
-struct sony_backlight_props sony_bl_props;
+static struct sony_backlight_props sony_bl_props;
 
 static int sony_backlight_update_status(struct backlight_device *bd)
 {