docs: don't download DTDs when building docs

Fixes:

| xsltproc /usr/share/xml/docbook/stylesheet/nwalsh/manpages/docbook.xsl `echo grml-crypt.8.txt |sed -e 's/.txt/.xml/'`
| error : Resource temporarily unavailable
| grml-crypt.8.xml:2: warning: failed to load external entity "http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd"
| D DocBook XML V4.5//EN" "http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd"
|       									 ^
| Note: Writing grml-crypt.8