VM Manager: Do not use [[nodiscard]] when compiling with GCC < 11, actually fixes #5707.

src/qt/qt_vmmanager_main.hpp

@@ -68,12 +68,12 @@ public slots:
     void shutdownForceButtonPressed() const;
     void searchSystems(const QString &text) const;
     void newMachineWizard();
-#ifdef _WIN32
     void addNewSystem(const QString &name, const QString &dir, const QString &configFile = {});
-#else
-    void addNewSystem(const QString &name, const QString &dir, const QString &configFile);
-#endif
+#if __GNUC__ >= 11
     [[nodiscard]] QStringList getSearchCompletionList() const;
+#else
+    QStringList getSearchCompletionList() const;
+#endif
     void modelDataChange();
     void onPreferencesUpdated();