Fix override error in system_clock.h (#10858)

Summary:
Fix error
```
 rocksdb/system_clock.h:30:11: error: '~SystemClock' overrides a destructor but is not marked 'override' [-Werror,-Wsuggest-destructor-override]
virtual ~SystemClock() {}
```

Pull Request resolved: https://github.com/facebook/rocksdb/pull/10858

Test Plan: Ran internally

Reviewed By: siying

Differential Revision: D40652374

Pulled By: akankshamahajan15

fbshipit-source-id: 5dda8ca03ea57d709442c87e23e5fe097d7db672

include/rocksdb/system_clock.h

@@ -27,7 +27,7 @@ struct ConfigOptions;
 // operating system time-related functionality.
 class SystemClock : public Customizable {
  public:
-  virtual ~SystemClock() {}
+  ~SystemClock() override {}
 
   static const char* Type() { return "SystemClock"; }
   static Status CreateFromString(const ConfigOptions& options,