cross-build: Add comment missing from 9e5b0d9eac5b

Whilst the commit message documented some of the details, I had intended
to include this comment in the actual header, but failed to amend the
commit properly.

Fixes: 9e5b0d9eac5b ("cross-build: Fix bmake bootstrap with glibc 2.34")
MFC after: 1 week

cross-build: Fix bmake bootstrap with glibc 2.34

As of glibc 2.34, our unistd.h wrapper's inclusion of stdlib.h exposes
fragility in glibc's sys/wait.h and corresponding part of stdlib.h,
leading to "error: use of undeclared identifier 'WNOHANG'" and similar
errors when bootstrapping bmake.

Work around this by wrapping sys/wait.h to force stdlib.h's inclusion
first before it's implicitly included during the problematic window in
sys/wait.h.

MFC after: 1 week