Avoid linker errors when including simple8b_rle.h/simple9_rle.h/vsencoding.h headers from multiple files

[lemire]

It seems that you are assuming C++17? See failed CI tests.

[pps83]

Yes, I see. I use c++20 in my code, so, it was ok on my side. The only way to make it work is to add .cpp files. Are you ok with that?

[pps83]

I think I have a better idea to fix it.

[pps83]

@lemire please recheck the PR

[lemire]

This should be fine. Thanks. Merging.