devel/spdlog: fix botched gmt offset calculation
The upstream code has like a handful of bugs in the place that should
obtain the GMT offset for a struct tm, and os-inl.h is a botch.
Strip the bogus workaround code and use tm.tm_gmtoff.
Reported by: diizzy@
URL: https://github.com/gabime/spdlog/issues/3483
Approved by: portmgr@ (just-fix-it blanket)
(based on commit 9f6a8a34ed864c5a40d987d6d102898ee124a821)