Fixing compiler warnings unused parameter.

variant.hpp

@@ -73,9 +73,9 @@ struct variant_helper<F, Ts...> {
 };
 
 template<> struct variant_helper<>  {
-  inline static void destroy(size_t id, void * data) { }
-  inline static void move(size_t old_t, void * old_v, void * new_v) { }
-  inline static void copy(size_t old_t, const void * old_v, void * new_v) { }
+  inline static void destroy(size_t, void *) { }
+  inline static void move(size_t, void *, void *) { }
+  inline static void copy(size_t, const void *, void *) { }
 };
 
 template <typename... Ts>