perf python: Add cgroup.c to setup.py to get it building again

perf python: Add cgroup.c to setup.py to get it building again

The 023695d cset added a new file, util/cgroup.c, that is referenced from
util/evsel.c, so it needs to be present in util/setup.py so that the python
shared object binding works, fixing this:

[root@emilia linux]# export PYTHONPATH=~acme/git/build/perf/python/
[root@emilia linux]# ./tools/perf/python/twatch.py
Traceback (most recent call last):
  File "./tools/perf/python/twatch.py", line 16, in <module>
    import perf
ImportError: /home/acme/git/build/perf/python/perf.so: undefined symbol: close_cgroup

Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Cc: Tom Zanussi <tzanussi@gmail.com>
LKML-Reference: <new-submission>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>