kbuild: docbook: specify KERNELDOC dependency correctly

It is not a good idea to describe

  %.xml: %.tmpl FORCE
    ...

and

  $(BOOKS): $(KERNELDOC)

separately. This cannot detect missing template files.

For example, add something to DOCBOOKS variable:
  DOCBOOKS += foobar.xml
and run
  make xmldocs

It will succeed even if Documention/DocBook/foobar.tmpl
does not exist.
Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Signed-off-by: Michal Marek <mmarek@suse.cz>

Documentation/DocBook/Makefile

@@ -88,13 +88,9 @@ define rule_docproc
         ) > $(dir $@).$(notdir $@).cmd
 endef
 
-%.xml: %.tmpl FORCE
+%.xml: %.tmpl $(KERNELDOC) $(DOCPROC) FORCE
 	$(call if_changed_rule,docproc)
 
-###
-# Changes in kernel-doc force a rebuild of all documentation
-$(BOOKS): $(KERNELDOC)
-
 # Tell kbuild to always build the programs
 always := $(hostprogs-y)