work around a swig issue on Arch Linux

destroy_atomic_prop_set() takes a parameter named 'as', and aparently
Swig reuses this name as-is, although it is a Python keyword.

* src/ltlvisit/apcollect.hh (destroy_atomic_prop_set): Rename the
parameter to please Swig on Arch Linux.

src/ltlvisit/apcollect.hh

 // -*- coding: utf-8 -*-
-// Copyright (C) 2012, 2013, 2014 Laboratoire de Recherche et
+// Copyright (C) 2012, 2013, 2014, 2015 Laboratoire de Recherche et
 // Développement de l'Epita (LRDE).
 // Copyright (C) 2004, 2005  Laboratoire d'Informatique de Paris 6 (LIP6),
 // département Systèmes Répartis Coopératifs (SRC), Université Pierre
@@ -46,7 +46,7 @@ namespace spot
     /// \brief Destroy all the atomic propositions in an
     /// atomic_prop_set.
     SPOT_API void
-    destroy_atomic_prop_set(atomic_prop_set& as);
+    destroy_atomic_prop_set(atomic_prop_set& aprops);
 
     /// \brief Return the set of atomic propositions occurring in a formula.
     ///