perf stat: Set child_pid after perf_evlist__prepare_workload()

The commit acf2892 ("perf stat: Use perf_evlist__prepare/
start_workload()") converted to use the function but forgot to update
child_pid.  Fix it.

Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Cc: David Ahern <dsahern@gmail.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/r/1380531671-28076-1-git-send-email-namhyung@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/builtin-stat.c

@@ -457,6 +457,7 @@ static int __run_perf_stat(int argc, const char **argv)
 			perror("failed to prepare workload");
 			return -1;
 		}
+		child_pid = evsel_list->workload.pid;
 	}
 
 	if (group)