docbook: need xmldoclinks for all doc types

$ rm -rf build
$ mkdir build
$ cp .config build
$ make O=build htmldocs
...
xmlto: linux-2.6/build/Documentation/DocBook/media.xml
does not validate (status 3)
xmlto: Fix document syntax or use --skip-validation option
linux-2.6/build/Documentation/DocBook/media.xml:4:
warning: failed to load external entity
"linux-2.6/build/Documentation/DocBook/media-entities.tmpl"

We need the xmldoclinks built for any document types built from the
XML sources.

Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Acked-by: Andy Whitcroft <apw@canonical.com>
Cc: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>