Add CONFIG_CPUSETS to check-config.sh

See also https://bugs.gentoo.org/show_bug.cgi?id=536546:

```console
$ docker run learn/tutorial echo "hello world"
FATA[0001] Error response from daemon: Cannot start container 1d0c0ca5cc7f39a271ec40c1479a80bffb7190fab97392b3453a1fc0b2dc8e78: mountpoint for cpuset not found
```

Signed-off-by: Andrew "Tianon" Page <admwiggin@gmail.com>

contrib/check-config.sh

@@ -138,7 +138,7 @@ fi
 flags=(
 	NAMESPACES {NET,PID,IPC,UTS}_NS
 	DEVPTS_MULTIPLE_INSTANCES
-	CGROUPS CGROUP_CPUACCT CGROUP_DEVICE CGROUP_FREEZER CGROUP_SCHED
+	CGROUPS CGROUP_CPUACCT CGROUP_DEVICE CGROUP_FREEZER CGROUP_SCHED CPUSETS
 	MACVLAN VETH BRIDGE
 	NF_NAT_IPV4 IP_NF_FILTER IP_NF_TARGET_MASQUERADE
 	NETFILTER_XT_MATCH_{ADDRTYPE,CONNTRACK}