Fix classic Windows configuration for libexslt

As is, the `LIBEXSLT_DOTTED_VERSION` would be `@LIBEXSLT_VERSION@` when
using the classic Windows configuration (win32/configure.js).  This has
apparently been overlooked when exsltconfig.h.in has been changed a
while ago[1].

[1] <https://gitlab.gnome.org/GNOME/libxslt/-/commit/4045ed4b05bb400359d02ce6e9b0ca9b97c04750>