make exported function private in source

[skoppe]

Because of dynamic library support we need to export 2 functions that allow us to sync the globalStopSource and the TLS localThreadExecutor across host/libraries.

However, that makes these functions accessible in source code. They are marked export which conflicts with private.

In the words of Martin Kinkelin:

In theory, we could make it private and embed an MSVC linker directive (pragma(linkerDirective, "/EXPORT:")) for the export (+ Linux export in dub.sdl).

That would still make the symbol loadable but at least not directly in source code.