selftests: intel_pstate: ftime() is deprecated

Use clock_gettime() instead of deprecated ftime().

  aperf.c: In function ‘main’:
  aperf.c:58:2: warning: ‘ftime’ is deprecated [-Wdeprecated-declarations]
     58 |  ftime(&before);
        |  ^~~~~
  In file included from aperf.c:9:
  /usr/include/sys/timeb.h:39:12: note: declared here
     39 | extern int ftime (struct timeb *__timebuf)
        |            ^~~~~

Signed-off-by: Tommi Rantala <tommi.t.rantala@nokia.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>