Summary:
Hello Facebook,

This small PR to fix the redundant std::move usage detected by gcc (GCC) 9.0.1 20190225 (experimental)

```
/folly/BUILD/folly-2019.04.01.00/folly/executors/task_queue/PriorityLifoSemMPMCQueue.h:92:30: error: redundant move in return statement [-Werror=redundant-move]
   92 |         return std::move(item);
      |                              ^
```

Stac
Pull Request resolved: https://github.com/facebook/folly/pull/1107

Reviewed By: Orvid

Differential Revision: D14874144

Pulled By: yfeldblum

fbshipit-source-id: c65a422c500606103192b3d9377a52b4abf5a653