perf report: Show per-event LOST SAMPLES stat

Display lost samples with --stat (if not zero):

  $ perf report --stat
    Aggregated stats:
             TOTAL events:         64
              COMM events:          2  ( 3.1%)
              EXIT events:          1  ( 1.6%)
            SAMPLE events:         26  (40.6%)
             MMAP2 events:          4  ( 6.2%)
      LOST_SAMPLES events:          1  ( 1.6%)
              ATTR events:          2  ( 3.1%)
    FINISHED_ROUND events:          1  ( 1.6%)
          ID_INDEX events:          1  ( 1.6%)
        THREAD_MAP events:          1  ( 1.6%)
           CPU_MAP events:          1  ( 1.6%)
      EVENT_UPDATE events:          2  ( 3.1%)
         TIME_CONV events:          1  ( 1.6%)
           FEATURE events:         20  (31.2%)
     FINISHED_INIT events:          1  ( 1.6%)
  cycles:uH stats:
            SAMPLE events:         14
      LOST_SAMPLES events:          1
  instructions:uH stats:
            SAMPLE events:         12

Signed-off-by: Namhyung Kim <namhyung@kernel.org>