move allocMatFromBuf function to farneback.cpp:

* it is the only place, where it is used
* no need to make this function public

modules/core/include/opencv2/core/cuda.hpp

@@ -314,8 +314,6 @@ The function does not reallocate memory if the matrix has proper attributes alre
  */
 CV_EXPORTS void ensureSizeIsEnough(int rows, int cols, int type, OutputArray arr);
 
-CV_EXPORTS GpuMat allocMatFromBuf(int rows, int cols, int type, GpuMat& mat);
-
 //! BufferPool management (must be called before Stream creation)
 CV_EXPORTS void setBufferPoolUsage(bool on);
 CV_EXPORTS void setBufferPoolConfig(int deviceId, size_t stackSize, int stackCount);