perf pmu-events: Fix missing "cpu_clk_unhalted.core" event

The events defined in pmu-events JSON are parsed and added into perf
tool. For fixed counters, we handle the encodings between JSON and perf
by using a static array fixed[].

But the fixed[] has missed an important event "cpu_clk_unhalted.core".

For example, on the Tremont platform,

  [root@localhost ~]# perf stat -e cpu_clk_unhalted.core -a
  event syntax error: 'cpu_clk_unhalted.core'
                       \___ parser error

With this patch, the event cpu_clk_unhalted.core can be parsed.

  [root@localhost perf]# ./perf stat -e cpu_clk_unhalted.core -a -vvv
  ------------------------------------------------------------
  perf_event_attr:
    type                             4
    size                             112
    config                           0x3c
    sample_type                      IDENTIFIER
    read_format                      TOTAL_TIME_ENABLED|TOTAL_TIME_RUNNING
    disabled                         1
    inherit               ...