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