On process 1.6.27.0, sys/wait.h is included unconditionally which breaks builds on wasm32-wasi

[wz1000]

Commit 5c98f59 moved the #if defined(HAVE_FORK) guard in cbits/posix/runProcess.c but left
#include <sys/wait.h> outside the guard.

On wasm32-wasi, sys/wait.h does not exist and HAVE_FORK is not defined, causing a build failure.

This was found on the x86_64-linux-alpine3_23-wasm-cross_wasm32-wasi-release GHC CI job.