Add GtkSourceView4 library as a dependency

In the Inkscape MR inkscape!4885 (merged), @mkov and I are adding support for syntax highlighting in the XML Dialog. This feature will be unavailable when the gtksourceview4 library is not installed, so we mark it as a dependency.

The intention is to wait until the feature is merged into master at Inkscape before this CI change is made. For this reason, the linked MR is marked as a dependency of this one.