fix missing addref() in ocl::Context::create(str)
fix missing addref() in ocl::Context::create(str)

- fix https://github.com/opencv/opencv/issues/18906
- unable to add related test cases as there is
  no public access to Context:Impl refcounts

opencv/opencv,opencv/opencv,opencv/opencv,opencv/opencv,opencv/opencv,opencv/opencv,opencv/opencv,opencv/opencv,opencv/opencv,opencv/opencv