CppTools: Avoid compiler warning

Change-Id: I19c423dd9f6cbaecbaa925df0090af001b951601
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@digia.com>

src/plugins/cpptools/cppmodelmanager_test.cpp

@@ -1045,6 +1045,7 @@ void CppToolsPlugin::test_modelmanager_renameIncludes()
     struct ModelManagerGCHelper {
         ~ModelManagerGCHelper() { CppModelManager::instance()->GC(); }
     } GCHelper;
+    Q_UNUSED(GCHelper); // do not warn about being unused
 
     QTemporaryDir tmpDir;
     QVERIFY(tmpDir.isValid());