Restores the standard-out appender's thread safety.

Since commit 71218f708607e121f7097c740d850b76b83f655a `str-println`
no longer prints the newline with the other output, this is restored.

I don't know if the removal of `str` was originally intended, but it results in scrambled output (misplaced newlines) when logging with multiple threads to standard-out.