Only print timestamps on stderr if verbose messages will go there

Only print timestamps on stderr if verbose messages will go there

On Windows, dbus can be configured to send DBUS_VERBOSE messages
to the Windows debug port instead of stderr. If we're in that
configuration, we already avoided printing thread information to
stderr before each verbose message; do the same for timestamps.

Reviewed-by: Simon McVittie <simon.mcvittie@collabora.co.uk>
Bug: https://bugs.freedesktop.org/show_bug.cgi?id=99749