Fix clang warning in renderpass D3DX12 header

[walbourn]

Fixes these warnings that happens with this D3DX12 header with clang/LLVM:

Microsoft.Direct3D.D3D12.1.613.3\build\native\include\d3dx12/d3dx12_render_pass.h(51,13): warning : enumeration values 'D3D12_RENDER_PASS_BEGINNING_ACCESS_TYPE_DISCARD', 'D3D12_RENDER_PASS_BEGINNING_ACCESS_TYPE_PRESERVE', and 'D3D12_RENDER_PASS_BEGINNING_ACCESS_TYPE_NO_ACCESS' not handled in switch [-Wswitch] 

Microsoft.Direct3D.D3D12.1.613.3\build\native\include\d3dx12/d3dx12_render_pass.h(70,13): warning : enumeration values 'D3D12_RENDER_PASS_ENDING_ACCESS_TYPE_DISCARD', 'D3D12_RENDER_PASS_ENDING_ACCESS_TYPE_PRESERVE', and 'D3D12_RENDER_PASS_ENDING_ACCESS_TYPE_NO_ACCESS' not handled in switch [-Wswitch]

[jenatali]

Please just add the default:. It's 2 lines and suppresses all of these warnings without compiler-specific #ifdef or #pragma shenanigans.

There's also MSVC 4061, which would show up here (if it was enabled) because it just unconditionally complains about enum switches with missing cases, even if there's a default.

[jenatali]

Clicked the wrong button with my previous comment...