tools/build: Bring in stdckdint.h if needed

This is needed when bootstrapping libc, reallocarray.c and
recallocarray.c include stdckdint.h now.

Reviewed by: emaste
Fixes: 7233893e9496 ("lib{c,openbsd}: use ckd_mul() for overflow checking in re(c)allocarray")
Differential Revision: https://reviews.freebsd.org/D52932