perf trace: No need to enable evsels for workload started from perf

As they will have perf_event_attr.enable_on_exec set, starting as soon
as we exec() the workload.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Borislav Petkov <bp@suse.de>
Cc: David Ahern <dsahern@gmail.com>
Cc: Don Zickus <dzickus@redhat.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/n/tip-vmj3f6o3vxrg7mrdipts09li@git.kernel.orgSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/builtin-trace.c

@@ -2109,10 +2109,10 @@ static int trace__run(struct trace *trace, int argc, const char **argv)
 	if (err < 0)
 		goto out_error_mmap;
 
-	perf_evlist__enable(evlist);
-
 	if (forks)
 		perf_evlist__start_workload(evlist);
+	else
+		perf_evlist__enable(evlist);
 
 	trace->multiple_threads = evlist->threads->map[0] == -1 || evlist->threads->nr > 1;
 again: