Fix "no new line at end of file" (#12806)

Summary:
internal CI with -Wnewline-eof complains about this.

Pull Request resolved: #12806

Reviewed By: jowlyzhang

Differential Revision: D58969551

Pulled By: cbi42

fbshipit-source-id: f97bd23f82129bac8dfd97b0ff4dbf7d2ded95d4

Author: cbi42

include/rocksdb/user_write_callback.h

@@ -26,4 +26,4 @@ class UserWriteCallback {
   virtual void OnWalWriteFinish() = 0;
 };
 
-}  // namespace ROCKSDB_NAMESPACE
+}  // namespace ROCKSDB_NAMESPACE