c_api: fix IndexShards own_indices getter/setter name mismatch (#5165)

Summary:
`IndexShards_c.cpp` defines:

```cpp
DEFINE_GETTER(IndexShards, int, own_indices)
DEFINE_SETTER(IndexShards, int, own_indices)
```

while `IndexShards_c.h` defines
```cpp
FAISS_DECLARE_GETTER_SETTER(IndexShards, int, own_fields)
```

this mismatch causes neither `faiss_IndexShards_set_own_indices` and `faiss_IndexShards_set_own_fields` to be exported in the c_api. This patch completes the migration to `indices` instead of `fields` in the c_api.

Pull Request resolved: #5165

Reviewed By: mnorris11

Differential Revision: D103238014

Pulled By: trang-nm-nguyen

fbshipit-source-id: 5a972cc6b6611088f06e3388677e0cd10d9037c0

Author: Binozo

c_api/IndexShards_c.h

@@ -22,7 +22,7 @@ extern "C" {
 FAISS_DECLARE_CLASS_INHERITED(IndexShards, Index)
 FAISS_DECLARE_DESTRUCTOR(IndexShards)
 
-FAISS_DECLARE_GETTER_SETTER(IndexShards, int, own_fields)
+FAISS_DECLARE_GETTER_SETTER(IndexShards, int, own_indices)
 FAISS_DECLARE_GETTER_SETTER(IndexShards, int, successive_ids)
 
 int faiss_IndexShards_new(FaissIndexShards** p_index, idx_t d);