ExtensionSystem: Replace a deprecated endl

Change-Id: Ib6b297b0176518d3bc2db1e45476cc9e8c00b2e0
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

src/libs/extensionsystem/pluginmanager.cpp

@@ -1121,7 +1121,7 @@ static TestPlan generateCustomTestPlan(IPlugin *plugin,
                 << "\".\nAvailable functions:\n";
             for (const QString &f : testFunctionsOfPluginObject)
                 out << "  " << f << '\n';
-            out << endl;
+            out << '\n';
         }
     }