fix check-config.sh to check for CGROUP_PERF

ping @unclejack @LK4D4 
again

> On Aug 20, 2014, at 1:37 PM, Erik Hollensbe <github@hollensbe.org> wrote:
> 
> feh
>> On Aug 20, 2014, at 1:25 PM, Tianon Gravi <notifications@github.com <mailto:notifications@github.com>> wrote:
>> 
>> In contrib/check-config.sh:
>> 
>> > @@ -146,6 +146,7 @@ echo 'Optional Features:'
>> >  flags=(
>> >  	MEMCG_SWAP
>> >  	RESOURCE_COUNTERS
>> > +  CGROUP_PERF
>> s/  /\t/
>> —
>> Reply to this email directly or view it on GitHub <https://github.com/docker/docker/pull/7632/files#r16502461>.
>> 
>