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>