time.h: add timegm() to the C23 namespace

This function is part of ISO/IEC 9899:2024 (C23) and was forgotten in D47856.

Reviewed by: imp
Approved by: markj (mentor)
See also: D47856
Fixes: 59677aecb67bbedcfa2ee5d7d2b189193cdc4af7
MFC after: 1 month
Differential Revision: https://reviews.freebsd.org/D53951