Debugger: Don't accumulate return value displays

This fixes a regression introduced with d771ba89.

Change-Id: Icc44ef9ad41f9e5cdb3848fae7a1ba8af2626344
Reviewed-by: Ulf Hermann <ulf.hermann@qt.io>

src/plugins/debugger/watchhandler.cpp

@@ -2004,7 +2004,7 @@ void WatchHandler::notifyUpdateStarted(const UpdateParameters &updateParameters)
 {
     QStringList inames = updateParameters.partialVariables();
     if (inames.isEmpty())
-        inames.append("local");
+        inames = QStringList({ "local", "return" });
 
     auto marker = [](WatchItem *item) { item->outdated = true; };