wayland: Ensure we have a dnd-ask cursor

We use this for DND, so make sure that we fall back to some other
cursor if this one isn't present.

gdk/wayland/gdkcursor-wayland.c

@@ -100,6 +100,7 @@ static const struct {
   { "copy",         "dnd-copy" },
   { "move",         "dnd-move" },
   { "no-drop",      "dnd-none" },
+  { "dnd-ask",      "dnd-copy" }, /* not CSS, but we want to guarantee it anyway */
   { "not-allowed",  "crossed_circle" },
   { "grab",         "hand2" },
   { "grabbing",     "hand2" },