[Bug libstdc++/65928] std::find_if compilation errors

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65928

--- Comment #5 from Jonathan Wakely <redi at gcc dot gnu.org> ---
More precisely, <string> includes <bits/localefwd.h> which declares (but does
not define) the overload of std::isblank.