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>