ocl: Add a function to unload a run-time cached program

This function is the counterpart of "Context::getProg".
With this function, users have chance to unload a program
from global run-time cached programs, and save resource.

modules/core/include/opencv2/core/ocl.hpp

@@ -248,6 +248,7 @@ public:
     const Device& device(size_t idx) const;
     Program getProg(const ProgramSource& prog,
                     const String& buildopt, String& errmsg);
+    void unloadProg(Program& prog);
 
     static Context& getDefault(bool initialize = true);
     void* ptr() const;