nlohmann-json: Enable multiple headers option
nlohmann-json: Enable multiple headers option

This installs json_fwd.hpp file and others instead of a monolith header file.
Fixes https://github.com/msys2/MINGW-packages/issues/10902
stahta01/MINGW-packages,oscarfv/MINGW-packages,fabiangreffrath/MINGW-packages,oroppas/MINGW-packages,stahta01/MINGW-packages,oroppas/MINGW-packages,Alexpux/MINGW-packages,stahta01/MINGW-packages,JPeterMugaas/MINGW-packages,oroppas/MINGW-packages,fabiangreffrath/MINGW-packages,Alexpux/MINGW-packages,Alexpux/MINGW-packages,oscarfv/MINGW-packages,lhmouse/MINGW-packages,oroppas/MINGW-packages,Alexpux/MINGW-packages,stahta01/MINGW-packages,oscarfv/MINGW-packages,JPeterMugaas/MINGW-packages,stahta01/MINGW-packages,lhmouse/MINGW-packages,fabiangreffrath/MINGW-packages,lhmouse/MINGW-packages,fabiangreffrath/MINGW-packages,fabiangreffrath/MINGW-packages,JPeterMugaas/MINGW-packages,fabiangreffrath/MINGW-packages,oscarfv/MINGW-packages,lhmouse/MINGW-packages,JPeterMugaas/MINGW-packages,oscarfv/MINGW-packages,Alexpux/MINGW-packages,JPeterMugaas/MINGW-packages,JPeterMugaas/MINGW-packages,stahta01/MINGW-packages,lhmouse/MINGW-packages,oroppas/MINGW-packages,lhmouse/MINGW-packages,oroppas/MINGW-packages,oscarfv/MINGW-packages,Alexpux/MINGW-packages