mate.libmatekbd: Fix darwin build

https://hydra.nixos.org/build/219011673/nixlog/3

pkgs/desktops/mate/libmatekbd/default.nix

···
 , fetchurl
 , pkg-config
 , gettext
-, gtk3
+, gtk3-x11
 , libxklavier
 , mateUpdateScript
 }:
···
   ];
 
   buildInputs = [
-    gtk3
+    gtk3-x11
     libxklavier
   ];