common/perf.cc: output message on skip
common/perf.cc: output message on skip

When calling perf_print_all_counters() with perf counters disabled,
Outputs a message reminding user that perf counters are not enabled.

Signed-off-by: Alan Green <97e77871b5fcc4941bebceb4338c97c1fd149f8c@google.com>

google/CFU-Playground,google/CFU-Playground,google/CFU-Playground,google/CFU-Playground