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 <[e-mail]>
Acked-by: Mauro Carvalho Chehab <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

Documentation/DocBook/Makefile

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