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>

src/microsoft/clc/compute_test.cpp

@@ -25,7 +25,7 @@
 #include <stdint.h>
 #include <stdexcept>
 
-#include <d3d12.h>
+#include <directx/d3d12.h>
 #include <dxgi1_4.h>
 #include <gtest/gtest.h>
 #include <wrl.h>