msi-wmi: make needlessly global symbols static

backlight is needlessly defined global.
This patch makes the symbol static.

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Acked-by: Anisse Astier <anisse@astier.eu>
Signed-off-by: Matthew Garrett <mjg@redhat.com>

drivers/platform/x86/msi-wmi.c

@@ -57,7 +57,7 @@ static struct key_entry msi_wmi_keymap[] = {
 };
 static ktime_t last_pressed[ARRAY_SIZE(msi_wmi_keymap) - 1];
 
-struct backlight_device *backlight;
+static struct backlight_device *backlight;
 
 static int backlight_map[] = { 0x00, 0x33, 0x66, 0x99, 0xCC, 0xFF };