fix check-config.sh to check for CGROUP_PERF

ping @unclejack @LK4D4 
feh
> On Aug 20, 2014, at 1:25 PM, Tianon Gravi <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>.
>