`CD3DX12_VERSIONED_ROOT_SIGNATURE_DESC` is missing a `Init_1_2` member function and constructor

[phoekz]

This is a very minor nitpick, but CD3DX12_VERSIONED_ROOT_SIGNATURE_DESC under d3dx12_root_signature.h doesn't have a member function variant of Init_1_2 function, while 1_0 and 1_1 have one.

Similarly, a constructor for 1_2 is also missing.