GAPI: fix warnings in own::Mat default generated constructors/assign op

Author: anton-potapov

modules/gapi/include/opencv2/gapi/own/mat.hpp

@@ -124,11 +124,11 @@ namespace cv { namespace gapi { namespace own {
            data = ptr(roi.y, roi.x);
         }
 
-        Mat(Mat const& src) = default;
-        Mat(Mat&& src) = default;
+        Mat(Mat const& ) = default;
+        Mat(Mat&& ) = default;
 
-        Mat& operator=(Mat const& src) = default;
-        Mat& operator=(Mat&& src) = default;
+        Mat& operator=(Mat const& ) = default;
+        Mat& operator=(Mat&& ) = default;
 
         /** @brief Sets all or some of the array elements to the specified value.
         @param s Assigned scalar converted to the actual array type.