Turbostat: fix shadowing issue

Turbostat: fix shadowing issue

turbostat.c: In function ‘delta_thread’:
turbostat.c:460:26: error: declaration of ‘core_delta’ shadows a global
declaration [-Werror=shadow]
  const struct core_data *core_delta)
                          ^~~~~~~~~~
turbostat.c:163:4: note: shadowed declaration is here
 } *core_delta, *core_even, *core_odd;
    ^~~~~~~~~~