gnu: Remove librsvg@2.50 from the Xfce dependency graph.

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.

gnu/packages/gstreamer.scm

@@ -681,7 +681,7 @@ for the GStreamer multimedia library.")
        ("bzip2" ,bzip2)
        ("cairo" ,cairo)
        ("flac" ,flac)
-       ("librsvg" ,librsvg)
+       ("librsvg" ,(librsvg-for-system))
        ("glib" ,glib)
        ("glib-networking" ,glib-networking)
        ("glu" ,glu)

gnu/packages/gtk.scm

@@ -1417,7 +1417,7 @@ exceptions, macros, and a dynamic programming environment.")
                        ("libtool" ,libtool)
                        ("texinfo" ,texinfo)))
       (inputs `(("guile" ,guile-3.0)
-                ("librsvg" ,librsvg)
+                ("librsvg" ,(librsvg-for-system))
                 ("guile-lib" ,guile-lib)))        ;for (unit-test)
       (propagated-inputs `(("guile-cairo" ,guile-cairo)))
       (synopsis "Render SVG images using Cairo from Guile")

gnu/packages/xfce.scm

@@ -633,7 +633,7 @@ applications, and includes a search bar to search for applications.")
        ("pkg-config" ,pkg-config)))
     (inputs
      `(("garcon" ,garcon)
-       ("librsvg" ,librsvg)
+       ("librsvg" ,(librsvg-for-system))
        ("libwnck" ,libwnck)
        ("libx11" ,libx11)
        ("libxfce4ui" ,libxfce4ui)