perf tools: Initialize cpu set in pthread_attr_setaffinity_np feature test

Feature tests are compiled but not executed, however it might avoid a
future uninitialized variable warning, so initialize the cpu set.

Reported-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Stephane Eranian <eranian@google.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-tip-commits@vger.kernel.org
Link: http://lkml.kernel.org/r/54F41849.1010906@intel.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>