perf jevents: Remove unused variable

Address gcc warning:

  pmu-events/jevents.c: In function ‘save_arch_std_events’:
  pmu-events/jevents.c:417:15: warning: unused variable ‘sb’ [-Wunused-variable]
    struct stat *sb = data;
                 ^~

Signed-off-by: Zenghui Yu <yuzenghui@huawei.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: John Garry <john.garry@huawei.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: wanghaibin.wang@huawei.com
Link: http://lkml.kernel.org/r/1557919169-23972-1-git-send-email-yuzenghui@huawei.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>