perf bench sched-messaging: Plug memleak

Explicitly free the thread array ('pth_tab').

Signed-off-by: Davidlohr Bueso <davidlohr@hp.com>
Cc: Aswin Chandramouleeswaran <aswin@hp.com>
Cc: Hitoshi Mitake <mitake@dcl.info.waseda.ac.jp>
Cc: Jiri Olsa <jolsa@kernel.org>
Link: http://lkml.kernel.org/r/1402942467-10671-5-git-send-email-davidlohr@hp.com


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