Fix missing overload variable in Rocksdb ivf demo (facebookresearch#3326)

Summary:
**Bugs:**
When following rocksdb_ivf demo to build executable file, its output as:
```bash
faiss/demos/rocksdb_ivf/RocksDBInvertedLists.h:52:35: error: 'faiss::InvertedListsIterator* faiss_rocksdb::RocksDBInvertedLists::get_iterator(size_t) const' marked 'override', but does not override
   52 |     faiss::InvertedListsIterator* get_iterator(size_t list_no) const override;
      |                                   ^~~~~~~~~~~~
make[2]: *** [CMakeFiles/demo_rocksdb_ivf.dir/build.make:90: CMakeFiles/demo_rocksdb_ivf.dir/RocksDBInvertedLists.cpp.o] Error 1
```

**Solution:**
Add relevant variable `void* inverted_list_contex` corresponding `get_iterator`'s base virtual function.

Pull Request resolved: facebookresearch#3326

Reviewed By: mlomeli1, mdouze

Differential Revision: D55629580

Pulled By: algoriddle

fbshipit-source-id: a12fcacb483e0dd576411ad91a3dd1e0de94abec

Author: Warmchay

demos/rocksdb_ivf/RocksDBInvertedLists.cpp

@@ -101,7 +101,8 @@ void RocksDBInvertedLists::resize(size_t /*list_no*/, size_t /*new_size*/) {
 }
 
 InvertedListsIterator* RocksDBInvertedLists::get_iterator(
-        size_t list_no) const {
+        size_t list_no,
+        void* inverted_list_context) const {
     return new RocksDBInvertedListsIterator(db_.get(), list_no, code_size);
 }
 

demos/rocksdb_ivf/RocksDBInvertedLists.h

@@ -49,7 +49,9 @@ struct RocksDBInvertedLists : faiss::InvertedLists {
 
     void resize(size_t list_no, size_t new_size) override;
 
-    faiss::InvertedListsIterator* get_iterator(size_t list_no) const override;
+    faiss::InvertedListsIterator* get_iterator(
+            size_t list_no,
+            void* inverted_list_context) const override;
 
    private:
     std::unique_ptr<rocksdb::DB> db_;