Update os.h
Update os.h

fixed issue #246
godbyk/spdlog,hunter-packages/spdlog,COMBINE-lab/spdlog,hunter-packages/spdlog,hunter-packages/spdlog,icylord/spdlog,icylord/spdlog,COMBINE-lab/spdlog,mihadyuk/spdlog,COMBINE-lab/spdlog,mihadyuk/spdlog,mihadyuk/spdlog,godbyk/spdlog,godbyk/spdlog,icylord/spdlog