tools: Copy lib/rbtree.c to tools/lib/

So that we can remove kernel specific stuff we've been stubbing out via
a tools/include/linux/export.h that gets removed in this patch and to
avoid breakages in the future like the one fixed recently where
rcupdate.h started being used in rbtree.h.

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-rxuzfsozpb8hv1emwpx06rm6@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>