tools/power turbostat: Use sched_getcpu() instead of hardcoded cpu 0

Disabling cpu 0 results in an error

turbostat: /sys/devices/system/cpu/cpu0/topology/thread_siblings: open failed: No such file or directory

Use sched_getcpu() instead of a hardcoded cpu 0 to get the max cpu number.

Signed-off-by: Prarit Bhargava <prarit@redhat.com>
Signed-off-by: Len Brown <len.brown@intel.com>