xmto: fix missing DTD/entities errors

`xmlto` forces the `--nonet` flag on `xsltproc`, which means it throws up an
I/O error whenever it needs to hit the network to pull down any files. So we
patch out the forced flag, and additionally add `docbook-xsl` as a dependency,
which'll reduce the odds of having to hit the network in the first place.

Fixes #21040.

Signed-off-by: Adam Vandenberg <flangy@gmail.com>