SuperVersion is a struct, not a class (#294)

igorcanadi · web-flow · commit 68c758aecd9e · 2023-10-19T11:45:00.000+01:00
Summary:
```
./db/snapshot_impl.h:55:1: error: class 'SuperVersion' was previously declared as a struct; this is valid, but may result in linker errors under the Microsoft C++ ABI [-Werror,-Wmismatched-tags]
```

Test Plan:

Reviewers:
diff --git a/db/snapshot_impl.h b/db/snapshot_impl.h
@@ -52,7 +52,7 @@ class SnapshotImpl : public Snapshot {
 };
 
 // RocksDB-Cloud contribution begin
-class SuperVersion;
+struct SuperVersion;
 class ColumnFamilyData;
 class SuperSnapshotImpl : public SnapshotImpl {
  public:
