netfilter: ctnetlink: fix compilation warning after data race fixes in ct mark

All warnings (new ones prefixed by >>):

   net/netfilter/nf_conntrack_netlink.c: In function '__ctnetlink_glue_build':
>> net/netfilter/nf_conntrack_netlink.c:2674:13: warning: unused variable 'mark' [-Wunused-variable]
    2674 |         u32 mark;
         |             ^~~~

Fixes: 52d1aa8b

 ("netfilter: conntrack: Fix data-races around ct mark")
Reported-by: kernel test robot <lkp@intel.com>
Tested-by: Ivan Babrou <ivan@ivan.computer>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>