fixes #11152 - add docbook-xsl to installer build-depends
xsltproc was not able to resolve the XSL for docbook locally and tried retrieving it from the internet, leading to build errors in case of flakey connections.
Related issues
Bug #11152: a2x: ERROR: "xsltproc" ... returned non-zero exit status 5 during Debian package build
fixes #11152 - add docbook-xsl to installer build-depends
xsltproc was not able to resolve the XSL for docbook locally and tried
retrieving it from the internet, leading to build errors in case of
flakey connections.