perf time-utils: Adopt rdclock() from perf.h

Seems to be a better place for this function to live, further shrinking
the hodge-podge that perf.h was.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: https://lkml.kernel.org/n/tip-0zzt1u9rpyjukdy1ccr2u5r9@git.kernel.org


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>