firmware: dmi: struct dmi_header should be packed

Apparently the compiler does fine without it, but it feels safer and
clearer to add the missing attribute.
Signed-off-by: Jean Delvare <jdelvare@suse.de>

include/linux/dmi.h

@@ -75,7 +75,7 @@ struct dmi_header {
 	u8 type;
 	u8 length;
 	u16 handle;
-};
+} __packed;
 
 struct dmi_device {
 	struct list_head list;