gthreadpool: Mark an explicit leak as to be ignored

This thread is created once during the process’ lifetime and cannot be destroyed and recreated, as the thread scheduler settings might have changed since then.

Mark the leak as explicit, mostly for documentation purposes — but it might quieten some static analysers.

Signed-off-by: Philip Withnall pwithnall@endlessos.org

Coverity CID: #1474614