QsLog: add missing lock

The destList is accessed both from the main thread and logger's writer
thread.

Within PMP, this shouldn't matter in practice, because we're not adding
new destinations after sending the first log message.

external/qslog/QsLog.cpp

@@ -187,6 +187,7 @@ Logger::~Logger()
 
 void Logger::addDestination(DestinationPtr destination)
 {
+    QMutexLocker lock(&d->logMutex);
     assert(destination.data());
     d->destList.push_back(destination);
 }