netfilter: nft_ct: fix compilation warning if NF_CONNTRACK_MARK is not set

net/netfilter/nft_ct.c: In function 'nft_ct_set_eval':
net/netfilter/nft_ct.c:136:6: warning: unused variable 'value' [-Wunused-variable]
Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Kristian Evensen <kristian.evensen@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/nft_ct.c

@@ -133,7 +133,9 @@ static void nft_ct_set_eval(const struct nft_expr *expr,
 {
 	const struct nft_ct *priv = nft_expr_priv(expr);
 	struct sk_buff *skb = pkt->skb;
+#ifdef CONFIG_NF_CONNTRACK_MARK
 	u32 value = data[priv->sreg].data[0];
+#endif
 	enum ip_conntrack_info ctinfo;
 	struct nf_conn *ct;