build: add stddef.h to define ptraddr_t as required

Effort: CHERI upstreaming
Sponsored by: Innovate UK
Reviewed by: kib
Co-authored-by: Alfredo Mazzinghi <am2419@cl.cam.ac.uk>
Co-authored-by: Alex Richardson <Alexander.Richardson@cl.cam.ac.uk>
Differential Revision: https://reviews.freebsd.org/D55308