tools/power turbostat: return the exit status of a command

turbostat failed to return a non-zero exit status even though the
supplied command (turbostat <command>) failed.  Currently when turbostat
forks a command it returns zero instead of the actual exit status of the
command.  Modify the code to return the exit status.

Signed-off-by: David Arcari <darcari@redhat.com>
Acked-by: Len Brown <len.brown@intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>