perf tools: Fix misleading add event PMU debug message

Fix incorrect debug message:

   Attempting to add event pmu 'intel_pt' with '' that may result in
   non-fatal errors

which always appears with perf record -vv and intel_pt e.g.

    perf record -vv -e intel_pt//u uname

The message is incorrect because there will never be non-fatal errors.

Suppress the message if the PMU is 'selectable' i.e. meant to be
selected directly as an event.

Fixes: 4ac22b48

 ("perf parse-events: Make add PMU verbose output clearer")
Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Link: http://lore.kernel.org/lkml/20220411061758.2458417-1-adrian.hunter@intel.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>