scripts: Add metal platform to platform_dict

Available in Vulkan-Headers since 1.1.102.
diff --git a/scripts/common_codegen.py b/scripts/common_codegen.py
index aae3425..1c69e17 100644
--- a/scripts/common_codegen.py
+++ b/scripts/common_codegen.py
@@ -52,6 +52,7 @@
     'fuchsia' : 'VK_USE_PLATFORM_FUCHSIA',
     'ios' : 'VK_USE_PLATFORM_IOS_MVK',
     'macos' : 'VK_USE_PLATFORM_MACOS_MVK',
+    'metal' : 'VK_USE_PLATFORM_METAL_MVK',
     'vi' : 'VK_USE_PLATFORM_VI_NN',
     'wayland' : 'VK_USE_PLATFORM_WAYLAND_KHR',
     'win32' : 'VK_USE_PLATFORM_WIN32_KHR',