Named GraphCutSeamFinderBase to allow its usage in makePtr.

Sequel to bcb6f038.

modules/stitching/include/opencv2/stitching/detail/seam_finders.hpp

@@ -205,7 +205,7 @@ private:
 class CV_EXPORTS GraphCutSeamFinderBase
 {
 public:
-    enum { COST_COLOR, COST_COLOR_GRAD };
+    enum CostType { COST_COLOR, COST_COLOR_GRAD };
 };