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>