netfilter: nft_reject: fix compilation warning if NF_TABLES_IPV6 is disabled

net/netfilter/nft_reject.c: In function 'nft_reject_eval':
net/netfilter/nft_reject.c:37:14: warning: unused variable 'net' [-Wunused-variable]
Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/nft_reject.c

@@ -34,8 +34,9 @@ static void nft_reject_eval(const struct nft_expr *expr,
 			      const struct nft_pktinfo *pkt)
 {
 	struct nft_reject *priv = nft_expr_priv(expr);
+#if IS_ENABLED(CONFIG_NF_TABLES_IPV6)
 	struct net *net = dev_net((pkt->in != NULL) ? pkt->in : pkt->out);
-
+#endif
 	switch (priv->type) {
 	case NFT_REJECT_ICMP_UNREACH:
 		if (priv->family == NFPROTO_IPV4)