Makefile.in: Make TM_P_H depend on $(TREE_H) [PR111021]

As PR111021 shows, the below ${port}-protos.h include tree.h
for code_helper and tree_code:

  arm/arm-protos.h:#include "tree.h"
  cris/cris-protos.h:#include "tree.h" (H-P removed this in r14-3218)
  microblaze/microblaze-protos.h:#include "tree.h"
  rl78/rl78-protos.h:#include "tree.h"
  stormy16/stormy16-protos.h:#include "tree.h"

, when compiling build/gencondmd.cc, the include hierarchy
makes it depend on tm_p.h -> ${port}-protos.h -> tree.h,
which further includes (depends on) some files that are
generated during the building, such as: all-tree.def,
tree-check.h and so on.  The previous commit r14-3215
should already force build/gencondmd.cc to depend on
${TREE_H}, so the reported build failure should be gone.

But for a long term maintenance, especially one day some
build/xxx.cc requires tm_p.h but not recog.h, the ${TREE_H}
dependence could be missed and a build failure will show
up.  So this patch is to make TM_P_H depend on $(TREE_H),
any new build/xxx.cc depending on tm_p.h will be able to
consider ${TREE_H}.

It's tested with cross-builds for the affected ports with
steps:
 1) dropped the fix r14-3215;
 2) reproduced the build failure with serial build;
 3) applied this patch, serial built and verified all passed;
 4) added back r14-3215, serial built and verified all passed;

	PR bootstrap/111021

gcc/ChangeLog:

	* Makefile.in (TM_P_H): Add $(TREE_H) as dependence.