perf tools: Add external commands to list-cmds

The `perf --list-cmds` output prints only internal commands, although
there is no reason for that from users' perspective.

Adding the external commands to commands array with NULL function
pointer allows printing all perf commands while not changing the logic
of command handler selection.

Signed-off-by: Michael Petlan <mpetlan@redhat.com>
Acked-by: Ian Rogers <irogers@google.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Link: https://lore.kernel.org/r/20220404221541.30312-2-mpetlan@redhat.com


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