community/appstream: add docbook-xsl-nons to makedepends
Fix rebuild failure due to missing non-namepsaced version of docbook-xsl
for xsl parsing by explicitly including the -nons
subpackage. Error found on the 3.21 builders:
docs/meson.build:15:4: ERROR: Problem encountered: Unable to find
Docbook XSL stylesheets for man pages
Ref: #16579 (closed)