perf python: Fix argument name list of read_on_cpu()

perf python: Fix argument name list of read_on_cpu()

Mandatory arguments need to be present in the argument name list, as
well as optional arguments, otherwise python barfs:

	# ./python/twatch.py
	Traceback (most recent call last):
	  File "./python/twatch.py", line 41, in <module>
	    main()
	  File "./python/twatch.py", line 32, in main
	    event = evlist.read_on_cpu(cpu)
	RuntimeError: more argument specifiers than keyword list entries

Hence, add cpu to the name list.

Cc: David Ahern <daahern@cisco.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Stephane Eranian <eranian@google.com>
Cc: Tom Zanussi <tzanussi@gmail.com>
Link: http://lkml.kernel.org/r/1301588863-20210-1-git-send-email-fweisbec@gmail.com
Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>