config/Makefile is not included for the 'clean' target, so invoke the
config/feature-checks/Makefile 'clean' target from Makefile.perf.

Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Link: http://lkml.kernel.org/n/tip-sh2cGvmsjbrazarlqre7pVwt@git.kernel.orgSigned-off-by: NIngo Molnar <mingo@kernel.org>