Pi zero, Buster OPsys - makedev issue

[Flettie]

All going smoothly up until I get a well-know error makedev error response. The solution is given as ...

"If you get the makedev error, the problem is a missing function called makedev, not a program.
makedev() is actually a macro and you need to include the correct header for it.
At the top of mailbox.c add the following line. "

#include <sys/sysmacros.h>

My issue is as I'm not a coder how/where do I add the above line. Sorry need some spoon feeding here