CppEditor: Silence warning

Amends f477ede697.

Change-Id: I99b10d2f9ba460ee0a1fa828e1b19465077192ef
Reviewed-by: David Schulz <david.schulz@qt.io>

src/plugins/cppeditor/cppfilesettingspage.cpp

@@ -744,6 +744,8 @@ void setupCppFileSettings(ExtensionSystem::IPlugin &plugin)
 
 #ifdef WITH_TESTS
     plugin.addTestCreator([] { return new CppFileSettingsTest; });
+#else
+    Q_UNUSED(plugin)
 #endif
 }