We were using the include/linux/rbtree.h directly from the kernel,
which broke the build as soon as it started using rcupdate.h, to
avoid dragging the rcu header files into tools/, for which there is
no use so far, grab a copy of rbtree.h.

This is the minimal fix, later patches will copy as well lib/rbtree.c
and move rbtree.h into tools/include/, etc.

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-dfmuj0j63w4by7vhlh4hhn74@git.kernel.orgSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>