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 <[email protected]>
Acked-by: Ingo Molnar <[email protected]>
Cc: Frederic Weisbecker <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Jiri Olsa <[email protected]>
Cc: Namhyung Kim <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Stephane Eranian <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Arnaldo Carvalho de Melo <[email protected]>

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