Fix the missing cl_mem retain in opencl-af example
Fix the missing cl_mem retain in opencl-af example

arrayfire/arrayfire-rust,arrayfire/arrayfire-rust,arrayfire/arrayfire-rust,9prady9/arrayfire-rust,arrayfire/arrayfire_rust,9prady9/arrayfire-rust,9prady9/arrayfire-rust,9prady9/arrayfire-rust