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)