kripke: avoir internal compiler error

gcc snapshot yield internal compiler error: tree check: accessed elt 2
of tree_vec with 1 elts in tsubst, at cp/pt.c:13693

* spot/kripke/kripke.hh: here.

spot/kripke/kripke.hh

@@ -67,7 +67,7 @@ namespace spot
       &kripkecube<State, SuccIter>::initial;
     std::string (kripkecube<State, SuccIter>::*test_to_string)
       (const State, unsigned) const = &kripkecube<State, SuccIter>::to_string;
-    auto (kripkecube<State, SuccIter>::*test_recycle)(SuccIter*, unsigned) =
+    void (kripkecube<State, SuccIter>::*test_recycle)(SuccIter*, unsigned) =
       &kripkecube<State, SuccIter>::recycle;
     const std::vector<std::string>
       (kripkecube<State, SuccIter>::*test_get_ap)() =