c:h export rocksdb_transactiondb_open_column_families (#7967)

Summary:
Hi, I noticed a bug in rocksdb C API, where a function is not exported and created a fix.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/7967

Reviewed By: jay-zhuang

Differential Revision: D26505722

Pulled By: ajkr

fbshipit-source-id: 05d676dbd59ec87fe32322cda9e39e405b07178d

include/rocksdb/c.h

@@ -1937,7 +1937,7 @@ extern ROCKSDB_LIBRARY_API rocksdb_transactiondb_t* rocksdb_transactiondb_open(
     const rocksdb_transactiondb_options_t* txn_db_options, const char* name,
     char** errptr);
 
-rocksdb_transactiondb_t* rocksdb_transactiondb_open_column_families(
+extern ROCKSDB_LIBRARY_API rocksdb_transactiondb_t* rocksdb_transactiondb_open_column_families(
     const rocksdb_options_t* options,
     const rocksdb_transactiondb_options_t* txn_db_options, const char* name,
     int num_column_families, const char* const* column_family_names,