openvswitch: Fix conntrack compilation without mark.

Fix build with !CONFIG_NF_CONNTRACK_MARK && CONFIG_OPENVSWITCH_CONNTRACK

Fixes: 182e3042 ("openvswitch: Allow matching on conntrack mark")
Reported-by: Simon Horman <simon.horman@netronome.com>
Signed-off-by: Joe Stringer <joestringer@nicira.com>
Tested-by: Simon Horman <simon.horman@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>