shotwell: Fix missing SVG icons

Shotwell relies on gdk-pixbuf to render SVG icons, but was missing the
librsvg loader at runtime.
Adding librsvg to the extraLoaders fixes this issue.

pkgs/by-name/sh/shotwell/package.nix

@@ -93,6 +93,7 @@ stdenv.mkDerivation (finalAttrs: {
       gnome._gdkPixbufCacheBuilder_DO_NOT_USE {
         extraLoaders = [
           libheif.lib
+          librsvg
         ];
       }
     }"