d3d12: Only support DXGI and GDI APIs on Windows

Reviewed-by: Erik Faye-Lund <erik.faye-lund@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/7937>

src/gallium/drivers/d3d12/d3d12_screen.cpp

@@ -633,9 +633,12 @@ d3d12_flush_frontbuffer(struct pipe_screen * pscreen,
       winsys->displaytarget_unmap(winsys, res->dt);
    }
 
+#ifdef _WIN32
+   // WindowFromDC is Windows-only, and this method requires an HWND, so only use it on Windows
    ID3D12SharingContract *sharing_contract;
    if (SUCCEEDED(screen->cmdqueue->QueryInterface(IID_PPV_ARGS(&sharing_contract))))
       sharing_contract->Present(d3d12_res, 0, WindowFromDC((HDC)winsys_drawable_handle));
+#endif
 
    winsys->displaytarget_display(winsys, res->dt, winsys_drawable_handle, sub_box);
 }