Fix os namespace in systemd_sink.h

[MyNameIsLame]

#2619 added os::thread_id() function calls to spdlog/sinks/systemd_sink.h.

But access to os::thread_id() is not valid, as the full namespace is spdlog::details::os, while the current code is in the namespace spdlog::sinks and thus the namespace os:: is not directly accessible.

As a side note: The test_systemd.cpp is currently not built in the pipeline, because it fails to find systemd in the cmake configuration step. Mabye it is sufficient to install the libsystemd-dev package in the container?

[gabime]

Thanks. I think details::os:: is enough and the spdlog:: can be omitted.
Regarding the fix to the CI, PR is welcome.

[MyNameIsLame]

Removed the spdlog:: prefix and also created #2669

[gabime]

Thanks @MyNameIsLame