tools build: Fix perf_clean target

Fix perf_clean target to follow the same logic as perf target.

Fixes the following make invokation:

  $ cd <kernelsrc> && make tools/perf_clean

Reported-by: TJ <linux@iam.tj>
Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=116411
Link: http://lkml.kernel.org/r/1461615438-27894-2-git-send-email-jolsa@kernel.org


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>