Include <memory> for std::unique_ptr

LLVM's libc++ is removing transitive inclusions among std header files in newer C++ versions, so user code must explicitly include needed std header files. In this case, cubeb_opensl.cpp must explicitly include <memory> for `std::unique_ptr` because, starting in C++23, <vector> no longers includes <memory>.

https://libcxx.llvm.org/DesignDocs/HeaderRemovalPolicy.html

```
libcubeb/src/cubeb_opensl.cpp:140:8: error: no template named 'unique_ptr' in namespace 'std'
  140 |   std::unique_ptr<cubeb_stream_params> input_params;
      |   ~~~~~^
```

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