metal: add missing copyright and license information

src/video_core/renderer_metal/metal_cpp_impl.cpp

@@ -1,3 +1,6 @@
+// SPDX-FileCopyrightText: Copyright 2024 suyu Emulator Project
+// SPDX-License-Identifier: GPL-2.0-or-later
+
 #define NS_PRIVATE_IMPLEMENTATION
 #define CA_PRIVATE_IMPLEMENTATION
 #define MTL_PRIVATE_IMPLEMENTATION