docbook: fixup media support files for htmldocs also

'make htmldocs' produces errors due to missing a supporting media
file, so add 'xmldoclinks' to the htmldocs dependencies so that the
needed supporting file will be present.

  Documentation/DocBook/media.xml:4: warning: failed to load external entity "Documentation/DocBook/media-entities.tmpl"
Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Acked-by: Mauro Carvalho Chehab <mchehab@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Documentation/DocBook/Makefile

@@ -45,7 +45,7 @@ PDF := $(patsubst %.xml, %.pdf, $(BOOKS))
 pdfdocs: $(PDF)
 
 HTML := $(sort $(patsubst %.xml, %.html, $(BOOKS)))
-htmldocs: $(HTML)
+htmldocs: $(HTML) xmldoclinks
 	$(call build_main_index)
 	$(call build_images)