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>

Makefile

@@ -247,6 +247,9 @@ $(GUEST_INIT_S2): guest/init_stage2.c
 
 $(DEPS):
 
+util/rbtree.d: ../../lib/rbtree.c
+	$(Q) $(CC) -M -MT util/rbtree.o $(CFLAGS) $< -o $@
+
 %.d: %.c
 	$(Q) $(CC) -M -MT $(patsubst %.d,%.o,$@) $(CFLAGS) $< -o $@