Add override to IDocument hierarchy

Change-Id: I6884f59fe0f06e380254c1f8076dd561d9df4ee3
Reviewed-by: Tobias Hunger <tobias.hunger@theqtcompany.com>

src/plugins/coreplugin/idocument.h

@@ -86,7 +86,7 @@ public:
     };
 
     IDocument(QObject *parent = 0);
-    virtual ~IDocument();
+    ~IDocument() override;
 
     void setId(Id id);
     Id id() const;