more final keywords

* src/ltlvisit/mutation.cc, src/ltlvisit/relabel.cc,
src/ltlvisit/remove_x.cc: Add final keyword to final classes.

src/ltlvisit/mutation.cc

@@ -46,7 +46,7 @@ namespace spot
   {
     namespace
     {
-      class replace_visitor : public clone_visitor
+      class replace_visitor final : public clone_visitor
       {
       public:
 	void visit(const atomic_prop* ap)
@@ -73,7 +73,7 @@ namespace spot
       };
 
       typedef std::vector<const formula*> vec;
-      class mutation_visitor : public clone_visitor
+      class mutation_visitor final : public clone_visitor
       {
       public:
         mutation_visitor(const formula* f, unsigned opts) : f_(f), opts_(opts)

src/ltlvisit/relabel.cc

@@ -45,7 +45,7 @@ namespace spot
 	virtual ~ap_generator() {}
       };
 
-      struct pnn_generator: ap_generator
+      struct pnn_generator final: ap_generator
       {
 	unsigned nn;
 	pnn_generator()
@@ -61,7 +61,7 @@ namespace spot
 	}
       };
 
-      struct abc_generator: ap_generator
+      struct abc_generator final: ap_generator
       {
       public:
 	abc_generator()
@@ -446,7 +446,7 @@ namespace spot
       }
 
 
-      class bse_relabeler: public relabeler
+      class bse_relabeler final: public relabeler
       {
       public:
 	fset& c;

src/ltlvisit/remove_x.cc

@@ -36,7 +36,7 @@ namespace spot
 #define G(x)      unop::instance(unop::G, (x))
 #define U(x, y)   binop::instance(binop::U, (x), (y))
 
-      class remove_x_visitor : public clone_visitor
+      class remove_x_visitor final : public clone_visitor
       {
 	typedef clone_visitor super;
 	atomic_prop_set aps;