fix check-config.sh to check for CGROUP_PERF

Docker-DCO-1.1-Signed-off-by: Erik Hollensbe <github@hollensbe.org> (github: erikh)

contrib/check-config.sh

@@ -146,6 +146,7 @@ echo 'Optional Features:'
 flags=(
 	MEMCG_SWAP
 	RESOURCE_COUNTERS
+	CGROUP_PERF
 )
 check_flags "${flags[@]}"