windowmaker: enable for darwin

It builds fine and I am able to run it through XQuartz server.

Signed-off-by: Ihar Hrachyshka <[e-mail]>

pkgs/by-name/wi/windowmaker/package.nix

···
     license = lib.licenses.gpl2Plus;
     mainProgram = "wmaker";
     maintainers = [ ];
-    platforms = lib.platforms.linux;
+    platforms = lib.platforms.unix;
   };
 })