xt-compat: constify a few struct members

avoids a compiler warning in a followup patch.

Signed-off-by: Florian Westphal <fw@strlen.de>
diff --git a/iptables/nft-shared.h b/iptables/nft-shared.h
index c0948fd..e13a1a8 100644
--- a/iptables/nft-shared.h
+++ b/iptables/nft-shared.h
@@ -223,9 +223,9 @@
 	unsigned int			command;
 	unsigned int			rulenum;
 	char				*table;
-	char				*chain;
-	char				*newname;
-	char				*policy;
+	const char			*chain;
+	const char			*newname;
+	const char			*policy;
 	bool				restore;
 	int				verbose;
 };