Make the option structures const.

Signed-off-by: Jan Engelhardt <jengelh@gmx.de>
diff --git a/extensions/libipt_dscp_helper.c b/extensions/libipt_dscp_helper.c
index fb68bb9..34aa1d3 100644
--- a/extensions/libipt_dscp_helper.c
+++ b/extensions/libipt_dscp_helper.c
@@ -14,7 +14,7 @@
 #include <xtables.h>
 
 
-static struct ds_class
+static const struct ds_class
 {
 	const char *name;
 	unsigned int dscp;