perf stat: Use strerror_r instead of strerror

Use strerror_r instead of strerror in error message for thread-safety.

Signed-off-by: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Naohiro Aota <naota@elisp.net>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/20140814022255.3545.81549.stgit@kbuild-fedora.novalocal


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