perf tools: Fix the detached tarball wrt rbtree copy

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 <[email protected]>
Cc: Borislav Petkov <[email protected]>
Cc: David Ahern <[email protected]>
Cc: Frederic Weisbecker <[email protected]>
Cc: Jiri Olsa <[email protected]>
Cc: Namhyung Kim <[email protected]>
Cc: Stephane Eranian <[email protected]>
Link: http://lkml.kernel.org/n/[email protected]
Signed-off-by: Arnaldo Carvalho de Melo <[email protected]>