Fix data race in accessing QDBusConnectionPrivate::serviceNames

The trick of creating a copy is not thread-safe. I'd known this since
the moment I wrote that code, but thought "what could go wrong?".

Task-number: QTBUG-39285
Change-Id: If521d4a649c06e6a34926687e85623aa25cb4c35
Reviewed-by: David Faure <david.faure@kdab.com>