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>