perf list: Add usage

Currently 'perf list' is not very helpful if you forget the syntax:

  $ perf list -h

  List of pre-defined events (to be used in -e):

After:
  $ perf list -h

   usage: perf list [hw|sw|cache|tracepoint|pmu|event_glob]

Signed-off-by: David Ahern <dsahern@gmail.com>
Acked-by: Ingo Molnar <mingo@kernel.org>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/r/527133AD.4030003@gmail.com


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