stddef.h: add ptraddr_t

I'd missed that stddef.h is standalone and isn't a copy of sys/stddef.h
in my initial merge.

Effort: CHERI upstreaming
Reviewed by: kib
Sponsored by: Innovate UK
Fixes: dca634d1544b ("new type: ptraddr_t")
Differential Revision: https://reviews.freebsd.org/D55305