perf python: Include rwsem.c in the pythong biding

We'll need it for the cgroup patches, and its better to have it in a
separate patch in case we need to later revert the cgroup patches.

I.e. without this we have:

  [root@five ~]# perf test -v python
  19: 'import perf' in python                               :
  --- start ---
  test child forked, pid 148447
  Traceback (most recent call last):
    File "<stdin>", line 1, in <module>
  ImportError: /tmp/build/perf/python/perf.cpython-37m-x86_64-linux-gnu.so: undefined symbol: down_write
  test child finished with -1
  ---- end ----
  'import perf' in python: FAILED!
  [root@five ~]#

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lore.kernel.org/lkml/20200403123606.GC23243@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/util/python-ext-sources

@@ -34,3 +34,4 @@ util/string.c
 util/symbol_fprintf.c
 util/units.c
 util/affinity.c
+util/rwsem.c