Add D3D_SVC_BIT_FIELD to D3D_SHADER_VARIABLE_CLASS

[python3kgae]

This is following up for https://github.com/microsoft/DirectXShaderCompiler/pull/5142

D3D_SVC_BIT_FIELD is added as macro define in https://github.com/microsoft/DirectXShaderCompiler/pull/5142

It should be declared in D3D_SHADER_VARIABLE_CLASS

Note: issue here is that DXC is essentially casting an integer value to D3D_SHADER_VARIABLE_CLASS becuase the D3D headers haven't been updated with the new value. So this is tracking work to modify the D3D headers appropriately.