QGtk3Menu: fix compilation with Clang 3.8

Clang complained about a missing override:

  qgtk3menu.h:58:14: error: 'tag' overrides a member function but is not marked 'override' [-Werror,-Winconsistent-missing-override]

Change-Id: I681cdeb93d35848ae39c1af289973b436e8ea60e
Reviewed-by: Giuseppe D'Angelo <giuseppe.dangelo@kdab.com>

src/plugins/platformthemes/gtk3/qgtk3menu.h

@@ -55,7 +55,7 @@ public:
     GtkWidget *create();
     GtkWidget *handle() const;
 
-    quintptr tag() const;
+    quintptr tag() const override;
     void setTag(quintptr tag) override;
 
     QString text() const;