[d3d11] Add fallthrough comment to PickFormats

Silences a warning

src/d3d11/d3d11_swapchain.cpp

@@ -588,7 +588,8 @@ namespace dxvk {
     switch (Format) {
       default:
         Logger::warn(str::format("D3D11SwapChain: Unexpected format: ", m_desc.Format));
-        
+      [[fallthrough]];
+      
       case DXGI_FORMAT_R8G8B8A8_UNORM:
       case DXGI_FORMAT_B8G8R8A8_UNORM: {
         pDstFormats[n++] = { VK_FORMAT_R8G8B8A8_UNORM, VK_COLOR_SPACE_SRGB_NONLINEAR_KHR };