V4L/DVB (13680b): DocBook/media: create links for included sources

If docs are being built in a separate directory, xmlto and xsltproc
can't find included sources.  Make links back to the source directory.

I would much prefer to have xmlto and xsltproc look in the source
directory for included entities but couldn't see how to do that.  This
needs to be solved in some way for 2.6.32, even if this patch isn't the
right way to do it.

Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>