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 <[e-mail]>
Acked-by: Andy Whitcroft <[e-mail]>
Cc: Randy Dunlap <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

Documentation/DocBook/Makefile

···
 PHONY += xmldocs sgmldocs psdocs pdfdocs htmldocs mandocs installmandocs cleandocs xmldoclinks
 
 BOOKS := $(addprefix $(obj)/,$(DOCBOOKS))
-xmldocs: $(BOOKS) xmldoclinks
+xmldocs: $(BOOKS)
 sgmldocs: xmldocs
 
 PS := $(patsubst %.xml, %.ps, $(BOOKS))
···
         ) > $(dir $@).$(notdir $@).cmd
 endef
 
-%.xml: %.tmpl FORCE
+%.xml: %.tmpl xmldoclinks FORCE
 	$(call if_changed_rule,docproc)
 
 ###