d/control: Add XSBC-Original-Vcs-* Fields

This makes sure we follow the practices documented under
https://tracker.pureos.net/w/development/packaging_overview/

debian/control

@@ -29,6 +29,8 @@ Rules-Requires-Root: no
 Standards-Version: 4.6.0
 Vcs-Browser: https://salsa.debian.org/gnome-team/gi-docgen
 Vcs-Git: https://salsa.debian.org/gnome-team/gi-docgen.git
+XSBC-Original-Vcs-Browser: https://salsa.debian.org/gnome-team/gi-docgen
+XSBC-Original-Vcs-Git: https://salsa.debian.org/gnome-team/gi-docgen.git
 Homepage: https://gitlab.gnome.org/GNOME/gi-docgen/
 
 Package: gi-docgen