perf utils: Silence "Couldn't synthesize bpf events" warning for EPERM

Synthesizing BPF events is only supported for root. Silent warning msg
when non-root user runs perf-record.

Reported-by: David Carrillo-Cisneros <davidca@fb.com>
Signed-off-by: Song Liu <songliubraving@fb.com>
Tested-by: David Carrillo-Cisneros <davidca@fb.com>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Cc: kernel-team@fb.com
Link: http://lkml.kernel.org/r/20190204193140.719740-1-songliubraving@fb.com


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