Merge pull request #42066 from thaJeztah/20.10_backport_check_config

[20.10 backport] check-config.sh: add NETFILTER_XT_MARK

contrib/check-config.sh

@@ -198,6 +198,7 @@ flags=(
 	VETH BRIDGE BRIDGE_NETFILTER
 	IP_NF_FILTER IP_NF_TARGET_MASQUERADE
 	NETFILTER_XT_MATCH_{ADDRTYPE,CONNTRACK,IPVS}
+	NETFILTER_XT_MARK
 	IP_NF_NAT NF_NAT
 
 	# required for bind-mounting /dev/mqueue into containers