Add missing index type to C-API
Add missing index type to C-API

Summary:
When the `TwoLevelIndexSearch` was introduced, it wasn't added to
the C-API.
Closes https://github.com/facebook/rocksdb/pull/2395

Differential Revision: D5165127

Pulled By: maysamyabandeh

fbshipit-source-id: d077f16ab5646c18158d8202a33b0fd076c6c8ad

bbiao/rocksdb,bbiao/rocksdb,Andymic/rocksdb,vmx/rocksdb,Andymic/rocksdb,Andymic/rocksdb,Andymic/rocksdb,Andymic/rocksdb,SunguckLee/RocksDB,SunguckLee/RocksDB,Andymic/rocksdb,Andymic/rocksdb,vmx/rocksdb,SunguckLee/RocksDB,bbiao/rocksdb,bbiao/rocksdb,SunguckLee/RocksDB,vmx/rocksdb,Andymic/rocksdb,bbiao/rocksdb,SunguckLee/RocksDB,SunguckLee/RocksDB,vmx/rocksdb,bbiao/rocksdb,SunguckLee/RocksDB,bbiao/rocksdb,bbiao/rocksdb,vmx/rocksdb,vmx/rocksdb,vmx/rocksdb,vmx/rocksdb,SunguckLee/RocksDB