[Bug libstdc++/90590] enumeration value not handled in switch warning for std::ios_base::seek_dir

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=90590

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
This is not a libstdc++ bug, we're allowed to define whatever enumerators we
want as long as they use reserved names.