perf python: Link with sysfs.o

So that we fix this regression:

[root@sandy linux]# perf test -v 15
15: Try 'use perf' in python, checking link problems       :
--- start ---
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ImportError: /home/acme/git/build/perf/python/perf.so: undefined symbol: sysfs_find_mountpoint
---- end ----
Try 'use perf' in python, checking link problems: FAILED!
[root@sandy linux]#

Cc: David Ahern <dsahern@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Namhyung Kim <namhyung@gmail.com>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/n/tip-8pf64bsdywg1gl9m55ul77hg@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/util/python-ext-sources

@@ -18,4 +18,5 @@ util/cgroup.c
 util/debugfs.c
 util/rblist.c
 util/strlist.c
+util/sysfs.c
 ../../lib/rbtree.c