This allows us to build Xfce on non-x86_64 platforms.
* gnu/packages/gstreamer.scm (gst-plugins-good)[inputs]: Use
'librsvg-for-system' instead of 'librsvg'.
* gnu/packages/gtk.scm (guile-rsvg)[inputs]: Likewise.
* gnu/packages/xfce.scm (xfce4-xkb-plugin)[inputs]: Likewise.