[NETFILTER]: ctnetlink: fix compile failure with NF_CONNTRACK_MARK=n

  CC      net/netfilter/nf_conntrack_netlink.o
net/netfilter/nf_conntrack_netlink.c: In function 'ctnetlink_conntrack_event':
net/netfilter/nf_conntrack_netlink.c:392: error: 'struct nf_conn' has no member named 'mark'
make[3]: *** [net/netfilter/nf_conntrack_netlink.o] Error 1

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>