add BackgroundSubtractor(KNN|MOG2).getDefaultName() implementation

modules/video/src/bgfg_KNN.cpp

@@ -214,6 +214,8 @@ public:
         }
     }
 
+    virtual String getDefaultName() const CV_OVERRIDE { return "BackgroundSubtractor_KNN"; }
+
     virtual int getHistory() const CV_OVERRIDE { return history; }
     virtual void setHistory(int _nframes) CV_OVERRIDE { history = _nframes; }
 

modules/video/src/bgfg_gaussmix2.cpp

@@ -236,6 +236,8 @@ public:
         }
     }
 
+    virtual String getDefaultName() const CV_OVERRIDE { return "BackgroundSubtractor_MOG2"; }
+
     virtual int getHistory() const CV_OVERRIDE { return history; }
     virtual void setHistory(int _nframes) CV_OVERRIDE { history = _nframes; }