minor fixes in stitching module

modules/stitching/include/opencv2/stitching/blenders.hpp

@@ -51,6 +51,8 @@ namespace cv
 class CV_EXPORTS Blender
 {
 public:
+    virtual ~Blender() {}
+
     enum { NO, FEATHER, MULTI_BAND };
     static Ptr<Blender> createDefault(int type, bool try_gpu = false);
 

modules/stitching/include/opencv2/stitching/exposure_compensate.hpp

@@ -50,6 +50,8 @@ namespace cv
 class CV_EXPORTS ExposureCompensator
 {
 public:
+    virtual ~ExposureCompensator() {}
+
     enum { NO, GAIN, GAIN_BLOCKS };
     static Ptr<ExposureCompensator> createDefault(int type);
 

modules/stitching/include/opencv2/stitching/motion_estimators.hpp

@@ -53,6 +53,8 @@ namespace cv
 class CV_EXPORTS Estimator
 {
 public:
+    virtual ~Estimator() {}
+
     void operator ()(const std::vector<ImageFeatures> &features, const std::vector<MatchesInfo> &pairwise_matches, 
                      std::vector<CameraParams> &cameras)
     {