Fix the Qt5 build after d95ba75

Sorry, my own fault this time.

src/qt/bitcoin.cpp

@@ -134,7 +134,7 @@ void DebugMessageHandler(QtMsgType type, const QMessageLogContext& context, cons
 {
     Q_UNUSED(context);
     const char *category = (type == QtDebugMsg) ? "qt" : NULL;
-    LogPrint(category, "GUI: %s\n", QString::toStdString(msg));
+    LogPrint(category, "GUI: %s\n", msg.toStdString());
 }
 #endif