gnu: xfce4-settings: Add gsettings-desktop-schemas.

* gnu/packages/xfce.scm (xfce4-settings):[propagated-inputs]: Add
gsettings-desktop-schemas. This makes it appear in the system profile
via the xfce service. This fixes a bug where changing icon theme causes
xfce4-appearance-settings to crash with the error:

GLib-GIO-ERROR **: 15:41:45.828: Settings schema 'org.gnome.desktop.interface' is not installed
Trace/breakpoint trap

Signed-off-by: Ludovic Courtès <ludo@gnu.org>

gnu/packages/xfce.scm

@@ -756,6 +756,10 @@ allows you to shut down the computer from Xfce.")
        ("upower" ,upower) ;; TODO needs upower-glib
        ("python" ,python) ;; for xfce4-compose-mail
        ("xf86-input-libinput" ,xf86-input-libinput)))
+    (propagated-inputs
+     ;; Some operations, such as changing icon themes, require these schemas
+     ;; to be in the search path.
+     `(("gsettings-desktop-schemas" ,gsettings-desktop-schemas)))
     (home-page "https://www.xfce.org/")
     (synopsis "Xfce settings manager")
     (description