perf tools: Check existence of _get_comp_words_by_ref when bash completing

The '_get_comp_words_by_ref' function is available from the bash
completion v1.2 so that earlier version emits following warning:

  $ perf re<TAB>_get_comp_words_by_ref: command not found

Use older '_get_cword' method when the above function doesn't exist.

Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Acked-by: Frederic Weisbecker <fweisbec@gmail.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/1349191294-6926-2-git-send-email-namhyung@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>