test: example include review (thanks iwyu)

test/example/entity_copy.cpp

@@ -4,6 +4,8 @@
 #include <entt/entity/registry.hpp>
 #include <entt/entity/storage.hpp>
 #include <entt/meta/factory.hpp>
+#include <entt/meta/meta.hpp>
+#include <entt/meta/policy.hpp>
 #include <entt/meta/resolve.hpp>
 
 enum class my_entity : entt::id_type {};

test/example/signal_less.cpp

@@ -2,6 +2,7 @@
 #include <type_traits>
 #include <gtest/gtest.h>
 #include <entt/entity/entity.hpp>
+#include <entt/entity/mixin.hpp>
 #include <entt/entity/registry.hpp>
 
 template<typename Type, typename Entity>