wctype-h: Fix compilation errors in C++ (regression from 2020-01-25). Reported by Christian Biesinger in <https://lists.gnu.org/archive/html/bug-gnulib/2020-02/msg00064.html>. * lib/wctype.in.h (iswdigit, iswxdigit): Don't declare if the corresponding module is not enabled. * tests/test-wctype-h-c++.cc (iswdigit, iswxdigit): Don't check the prototype if the corresponding module is not enabled.