perf report: Use timestamp__scnprintf_nsec() for time sort key

Use timestamp__scnprintf_nsec() to print nanoseconds for the time sort
key, instead of open coding.

Signed-off-by: Andi Kleen <ak@linux.intel.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Link: http://lkml.kernel.org/r/20190823210338.12360-1-andi@firstfloor.org


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