[cpp] (minor) Closing namespace comment order reversed

[warning] This is very minor but I still want to report it ;P

Consider the following generated c++ source:
```c++
// headers and such...

namespace tink{
namespace tcp{
namespace uv{

// ...

} // end namespace tink
} // end namespace tcp
} // end namespace uv

```

Note that the "end namespace" order is reversed (outer namespace should end last)

I think the iteration here should be reversed:
https://github.com/HaxeFoundation/haxe/blob/c76e7b3db3d411694a763ddab2ca21c8096835e5/src/generators/gencpp.ml#L577-L580