perf: Remove duplicated workqueue.h include from perf_event.h

It is already included a little bit higher up in that file.

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/20190117072504.14428-1-yuehaibing@huawei.com


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