kvm-tools: Create util/rbtree.d

I found the program is always RE-created when I type 'make' even
it is new:

$ make
  LINK     lkvm
  LN       vm

I use 'make -d' to find the reason: rbtree.d is missing.

When I type 'make' after this patch applied(the lkvm is new):

$ make
make: Nothing to be done for `all'.

Acked-by: Sasha Levin <levinsasha928@gmail.com>
Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>