Add CONFIG_KEYS to check-config.sh

We need this after opencontainers/runc#488

Signed-off-by: Alexander Morozov <lk4d4@docker.com>

contrib/check-config.sh

@@ -182,6 +182,7 @@ flags=(
 	NAMESPACES {NET,PID,IPC,UTS}_NS
 	DEVPTS_MULTIPLE_INSTANCES
 	CGROUPS CGROUP_CPUACCT CGROUP_DEVICE CGROUP_FREEZER CGROUP_SCHED CPUSETS MEMCG
+	CONFIG_KEYS
 	MACVLAN VETH BRIDGE BRIDGE_NETFILTER
 	NF_NAT_IPV4 IP_NF_FILTER IP_NF_TARGET_MASQUERADE
 	NETFILTER_XT_MATCH_{ADDRTYPE,CONNTRACK}