Added final specifier back to the View class templates

See https://github.com/skypjack/entt/pull/4#issuecomment-304510644

src/registry.hpp

@@ -20,7 +20,7 @@ class View;
 
 
 template<template<typename...> class Pool, typename Entity, typename... Components, typename Type, typename... Types>
-class View<Pool<Entity, Components...>, Type, Types...> {
+class View<Pool<Entity, Components...>, Type, Types...> final {
     using pool_type = Pool<Entity, Components...>;
     using entity_type = typename pool_type::entity_type;
 
@@ -122,7 +122,7 @@ private:
 
 
 template<template<typename...> class Pool, typename Entity, typename... Components, typename Type>
-class View<Pool<Entity, Components...>, Type> {
+class View<Pool<Entity, Components...>, Type> final {
     using pool_type = Pool<Entity, Components...>;
     using entity_type = typename pool_type::entity_type;