build: explicitly require librsvg
build: explicitly require librsvg

This is now required, as we use many SVG assets.

https://bugzilla.gnome.org/show_bug.cgi?id=653040

Distrotech/gnome-themes-standard,magcius/gnome-themes-standard,magcius/gnome-themes-standard,cosimoc/gnome-themes-standard,DoctorJellyface/gnome-themes-standard,rgcjonas/adwaita-semidark,ChrisCummins/gnome-themes-standard,cosimoc/gnome-themes-standard,ChrisCummins/gnome-themes-standard,rgcjonas/adwaita-semidark,DoctorJellyface/gnome-themes-standard,rgcjonas/adwaita-semidark,cosimoc/gnome-themes-standard,magcius/gnome-themes-standard,cosimoc/gnome-themes-standard,Distrotech/gnome-themes-standard,ChrisCummins/gnome-themes-standard,rgcjonas/adwaita-semidark,ChrisCummins/gnome-themes-standard,ChrisCummins/gnome-themes-standard