act_connmark: Add missing dependency on NF_CONNTRACK_MARK

Depending on NETFILTER is not sufficient to ensure the presence of the
'mark' field in nf_conn, also needs to depend on NF_CONNTRACK_MARK.

Fixes: 22a5dc ("net: sched: Introduce connmark action")
Cc: Felix Fietkau <nbd@openwrt.org>
Cc: Jamal Hadi Salim <jhs@mojatatu.com>
Signed-off-by: Thomas Graf <tgraf@suug.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>