[Bug libstdc++/85670] `std::filesystem` does not compile on mingw-w64

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85670

--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Oh I see, that line isn't compiled except on Windows, and does need a
declaration of operator!=.

I've fixed it locally now anyway.