Fix incorrect include (#2324)

Summary:
Was breaking after upgrading compiler

Discovered in the brew recipe. The error was:

```
.../folly/concurrency/SingletonRelaxedCounter.h:71:12: error: no template named 'unordered_map' in namespace 'std'; did you mean 'unordered_set'?
      std::unordered_map<LocalLifetime*, CounterAndCache*> lifetimes;
      ~~~~~^~~~~~~~~~~~~
           unordered_set
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX13.1.sdk/usr/include/c++/v1/unordered_set:411:28: note: 'unordered_set' declared here
class _LIBCPP_TEMPLATE_VIS unordered_set
                           ^
1 error generated.
make[2]: *** [CMakeFiles/folly_base.dir/folly/settings/Settings.cpp.o] Error 1
make[1]: *** [CMakeFiles/folly_base.dir/all] Error 2
make: *** [all] Error 2
```

Pull Request resolved: #2324

Reviewed By: ot

Differential Revision: D64793116

Pulled By: Orvid

fbshipit-source-id: 1df8040210a9dcb90d11576c8628e35a297925d2

folly/concurrency/SingletonRelaxedCounter.h

@@ -19,7 +19,7 @@
 #include <array>
 #include <atomic>
 #include <type_traits>
-#include <unordered_set>
+#include <unordered_map>
 
 #include <folly/Likely.h>
 #include <folly/Portability.h>