rocksdb_transaction_get_for_update now exports (#6293)

Summary:
Added missing ROCKSDB_LIBRARY_API decorator to rocksdb_transaction_get_for_update.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/6293

Reviewed By: jay-zhuang

Differential Revision: D25234298

Pulled By: ajkr

fbshipit-source-id: 8a4817adaec1f445f338c8d8c59d3392925b5721

include/rocksdb/c.h

@@ -1981,7 +1981,7 @@ extern ROCKSDB_LIBRARY_API char* rocksdb_transaction_get_for_update(
     const char* key, size_t klen, size_t* vlen, unsigned char exclusive,
     char** errptr);
 
-char* rocksdb_transaction_get_for_update_cf(
+extern ROCKSDB_LIBRARY_API char* rocksdb_transaction_get_for_update_cf(
     rocksdb_transaction_t* txn, const rocksdb_readoptions_t* options,
     rocksdb_column_family_handle_t* column_family, const char* key, size_t klen,
     size_t* vlen, unsigned char exclusive, char** errptr);