TextEditor: fix crash while request follow symbol and typing on mac

Use a qobject_cast to prevent accessing a not available function.

Task-number: QTCREATORBUG-20910
Change-Id: If2aa6b7b120379681e9590db80904c45a6d9bd55
Reviewed-by: Christian Stenger <christian.stenger@qt.io>

src/plugins/cpptools/cppvirtualfunctionassistprovider.h

@@ -40,6 +40,7 @@ namespace CppTools {
 
 class CPPTOOLS_EXPORT VirtualFunctionAssistProvider : public TextEditor::IAssistProvider
 {
+    Q_OBJECT
 public:
     VirtualFunctionAssistProvider();