vtkDataReader: Fix compilation of ReadString for C++20
It was broken because of P0487R1 in c++20.
Note: there some discussion of the status of this proposal in the library mailing lists, it's possible this isn't actually a defect report and thus breakage (at least on microsoft's standard library) won't happen unless compiling with /std:c++latest or /std:c++20 (when available). WIP until that's resolved.
Fixes: #17594
Edited by Brad King