d3d12: Use DirectX-Headers wrap for d3d12.h

This does 2 things for us:
1. Allows us to compile-time depend on any features from new headers,
   instead of having to conditionally compile based on Windows SDK version.
2. Allows us to reference d3d12.h when compiling for non-Windows.

Reviewed-by: Bill Kristiansen <billkris@microsoft.com>
Reviewed-by: Dylan Baker <dylan@pnwbakers.com>
Reviewed-by: Erik Faye-Lund <erik.faye-lund@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/7937>

subprojects/DirectX-Headers.wrap

@@ -0,0 +1,5 @@
+[wrap-git]
+directory = DirectX-Headers-1.0
+
+url = https://github.com/microsoft/DirectX-Headers.git
+revision = head