Make the option structures const.

Signed-off-by: Jan Engelhardt <jengelh@gmx.de>
diff --git a/extensions/libxt_string.c b/extensions/libxt_string.c
index c58f481..ea0a2aa 100644
--- a/extensions/libxt_string.c
+++ b/extensions/libxt_string.c
@@ -44,7 +44,7 @@
 IPTABLES_VERSION);
 }
 
-static struct option opts[] = {
+static const struct option opts[] = {
 	{ "from", 1, 0, '1' },
 	{ "to", 1, 0, '2' },
 	{ "algo", 1, 0, '3' },