perf tools: Fix the detached tarball wrt rbtree copy

The python binding build process was still looking at the kernel
rbtree.c file, so, when doing a in-tree build it would work, but when
creating a tarball using tools/perf/MANIFEST as the contents list and
then trying to build the resulting detached sources, it failed.

Fix it by removing one level of indirection from rbtree.c in the
tools/perf/util/python-ext-sources file.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Borislav Petkov <bp@suse.de>
Cc: David Ahern <dsahern@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/n/tip-8u83c2k5guyhxdlkaaqis8k4@git.kernel.org


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>