libxslt lazy dependency does not work transitively

[ianprime0509]

ziglang/zig#20716

After clearing my Zig cache, I noticed that zig-gobject's build was broken due to xsltproc not being available, even though the xslt option was set to true. It seems this is because of the transitive lazy dependency (zig-gobject has a lazy dependency on zig-libxml2, which has a lazy dependency on libxslt).