d3d12: Implement D3D12SerializeVersionedRootSignature() on top of vkd3d_serialize_versioned_root_signature().

This raises the required version of vkd3d to 1.2.
Signed-off-by: Henri Verbeet <hverbeet@codeweavers.com>
Signed-off-by: Alexandre Julliard <julliard@winehq.org>