Changed deprecated auto_ptr in AudioDevice to unique_ptr

[JinLisek]

• Has this change been discussed on the forum or in an issue before?
  Yes, here: https://en.sfml-dev.org/forums/index.php?topic=24313.0
• Does the code follow the SFML Code Style Guide?
  I guess so.
• Have you provided some example/test code for your changes?
  None, I've just changed auto_ptr to unique_ptr. I haven't found any unit tests in the repo. I ran cmake with ninja and it compiled.
• If you have additional steps which need to be performed list them as tasks!
  None

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Description

I removed deprecated auto_ptr in AudioDevice, so it no longer produces warnings when compiling with -Werror. The fix is tiny, shouldn't change anything, also the pointer is not even really used, at least in my understanding.

Tasks

• Tested on Linux
  It compiles on Linux, I haven't tested it on different platforms
• Tested on Windows
• Tested on macOS
• Tested on iOS
• Tested on Android

How to test this PR?

I don't think it's possible to test, as the pointer is not even used.

[eXpl0it3r]

SFML 2.x is built around C++03 and for backwards compatibility we like to keep it that way.
Since std::unique_ptr was introduced with C++11, we unfortunately can't accept this PR at this point.

Additionally, we've already started to implement some changes for SFML 3, which among others, already replaced std::auto_ptr, see the unstable branch. 😉

Thanks for your contribution! 🙂