Add docbook-xsl dependency.

Fixes mxcl/homebrew#21040.

Without `docbook-xsl`, `xmlto` fails to build `pgbouncer`'s manpages, as `xsltproc` throws up an I/O error trying to download http://docbook.sourceforge.net/release/xsl/current/manpages/docbook.xsl (among, I believe, others).