Build failure in folly/concurrency/SingletonRelaxedCounter.h

[likan999]

The file uses std::unordered_map but doesn't include the corresponding header, causes build failure in some cases. Can someone help to make the one-line change that includes this header? I can't send the PR myself due to my employer's policy. Thanks.

[greauxd]

Sent through PR with required fix, waiting on CLA authentication before change can be reviewed and merged!