graphics_breakpoints_p: Add missing override specifier

src/citra_qt/debugger/graphics_breakpoints_p.h

@@ -23,7 +23,7 @@ public:
     int columnCount(const QModelIndex& parent = QModelIndex()) const override;
     int rowCount(const QModelIndex& parent = QModelIndex()) const override;
     QVariant data(const QModelIndex& index, int role = Qt::DisplayRole) const override;
-    Qt::ItemFlags flags(const QModelIndex &index) const;
+    Qt::ItemFlags flags(const QModelIndex &index) const override;
 
     bool setData(const QModelIndex& index, const QVariant& value, int role = Qt::EditRole) override;