Fix ocl::dft the compile warning on Linux

modules/ocl/src/fft.cpp

@@ -55,8 +55,9 @@ void cv::ocl::dft(const oclMat&, oclMat&, Size, int)
     CV_Error(CV_StsNotImplemented, "OpenCL DFT is not implemented");
 }
 namespace cv { namespace ocl {
-    void fft_teardown(){}
+    void fft_teardown();
 }}
+void cv::ocl::fft_teardown(){}
 #else
 #include "clAmdFft.h"
 namespace cv