xfce.libxfce4ui: Explicitly add perl

This builds because xfce.xfce4-dev-tools propagates intltool, which
propagates perl. But since Xfce is actively getting rid of intltool
let's get ready for it.

pkgs/desktops/xfce/core/libxfce4ui/default.nix

@@ -2,6 +2,7 @@
   mkXfceDerivation,
   lib,
   gobject-introspection,
+  perl,
   vala,
   libICE,
   libSM,
@@ -23,6 +24,7 @@ mkXfceDerivation {
 
   nativeBuildInputs = [
     gobject-introspection
+    perl
     vala
   ];