xdg-desktop-portal-xapp: Fix missing xapp schema

This is especially a problem on non-Cinnamon desktops (we have xapp
in cinnamon-gsettings-overrides packaging).

pkgs/by-name/xd/xdg-desktop-portal-xapp/package.nix

@@ -11,8 +11,8 @@
   gtk3,
   gsettings-desktop-schemas,
   mate,
-  xdg-desktop-portal,
   xapp,
+  xdg-desktop-portal,
 }:
 
 stdenv.mkDerivation rec {
@@ -39,6 +39,7 @@ stdenv.mkDerivation rec {
     gtk3
     gsettings-desktop-schemas # org.gnome.system.location
     mate.mate-desktop # org.mate.background
+    xapp # org.x.apps.portal
     xdg-desktop-portal
   ];