Fix wrong availability for signal.SIGCHLD (GH-23285) (#23426)

I believe this is a mistake. SIGCHLD is only available on Unix systems, not Windows.
(cherry picked from commit 4c24b08c)

Co-authored-by: Zhang Maiyun <myzhang1029@hotmail.com>

Co-authored-by: Zhang Maiyun <myzhang1029@hotmail.com>