The qemu-doc.{html,info,pdf,txt} depend on qemu-doc.texi and its
include files.  Except qemu-tech.texi is missing.  Has always been
missing as far as I can see.  Fix it.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20190715055736.15214-2-armbru@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Tested-by: Philippe Mathieu-Daudé <philmd@redhat.com>