Fix missing DTD/entities errors.

Fixes mxcl/homebrew#21040.

`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.