build: Correct Metal platform name

In `common_codegen.py`, the Metal platform was defined without the
"_EXT" suffix

Change-Id: Ied604c46b55c669567517f5b2b7613d8c7f8ae64
diff --git a/scripts/common_codegen.py b/scripts/common_codegen.py
index c739bfa..47bb632 100644
--- a/scripts/common_codegen.py
+++ b/scripts/common_codegen.py
@@ -52,7 +52,7 @@
     'fuchsia' : 'VK_USE_PLATFORM_FUCHSIA',
     'ios' : 'VK_USE_PLATFORM_IOS_MVK',
     'macos' : 'VK_USE_PLATFORM_MACOS_MVK',
-    'metal' : 'VK_USE_PLATFORM_METAL',
+    'metal' : 'VK_USE_PLATFORM_METAL_EXT',
     'vi' : 'VK_USE_PLATFORM_VI_NN',
     'wayland' : 'VK_USE_PLATFORM_WAYLAND_KHR',
     'win32' : 'VK_USE_PLATFORM_WIN32_KHR',