perf test vmlinux: Avoid printing headers for empty lists

perf test vmlinux: Avoid printing headers for empty lists

Before:

  # perf test -F -v 1
   1: vmlinux symtab matches kallsyms:
  --- start ---
<SNIP>
  WARN: Maps only in vmlinux:
   ffffffffb7d7d000-ffffffffb7eeaac8 117d000 [kernel].init.text
   ffffffffb7eeaac8-ffffffffc03ad000 12eaac8 [kernel].exit.text
  WARN: Maps in vmlinux with a different name in kallsyms:
  WARN: Maps only in kallsyms:
  ---- end ----
  vmlinux symtab matches kallsyms: Ok
  #

The two last WARN lines are now suppressed, since there are no such
cases detected.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Wang Nan <wangnan0@huawei.com>
Link: http://lkml.kernel.org/n/tip-9ww8uvzl682ykaw8ht1tozlr@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>