tools/power x86_energy_perf_policy: Fix argument parsing

The -w argument in x86_energy_perf_policy currently triggers an
unconditional segfault.

This is because the argument string reads: "+a:c:dD:E:e:f:m:M:rt:u:vw" and
yet the argument handler expects an argument.

When parse_optarg_string is called with a null argument, we then proceed to
crash in strncmp, not horribly friendly.

The man page describes -w as taking an argument, the long form
(--hwp-window) is correctly marked as taking a required argument, and the
code expects it.

As such, this patch simply marks the short form (-w) as requiring an
argument.

Signed-off-by: Zephaniah E. Loss-Cutler-Hull <zephaniah@gmail.com>
Signed-off-by: Len Brown <len.brown@intel.com>