[leveldb] Specifically export the WriteBatch::Handler inner class for Windows link

Windows linking visibility in shared libraries requires that inner classes are
specifically exported as visible, even if the containing class is exported.

PiperOrigin-RevId: 244886019

include/leveldb/write_batch.h

@@ -62,7 +62,7 @@ class LEVELDB_EXPORT WriteBatch {
   void Append(const WriteBatch& source);
 
   // Support for iterating over the contents of a batch.
-  class Handler {
+  class LEVELDB_EXPORT Handler {
    public:
     virtual ~Handler();
     virtual void Put(const Slice& key, const Slice& value) = 0;