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

include/rocksdb/c.h

@@ -638,6 +638,7 @@ extern ROCKSDB_LIBRARY_API void rocksdb_block_based_options_set_format_version(
 enum {
   rocksdb_block_based_table_index_type_binary_search = 0,
   rocksdb_block_based_table_index_type_hash_search = 1,
+  rocksdb_block_based_table_index_type_two_level_index_search = 2,
 };
 extern ROCKSDB_LIBRARY_API void rocksdb_block_based_options_set_index_type(
     rocksdb_block_based_table_options_t*, int);  // uses one of the above enums