The pgtk window system exists

Oh, and because we're looking at symbols member can just be memq, why
not.

Author: tecosaur

org-ol-tree.el

@@ -326,7 +326,7 @@ The outline chooses the theme based on the following criteria:
 
 To find out if Emacs is running in GUI mode, we query the variable
 `window-system'."
-  (member window-system '(x w32 ns)))
+  (memq window-system '(pgtk x w32 ns)))