Add default in switch (#8065)

Summary:
switch may not cover all branch in `db/c.cc`:

```c++
void rocksdb_options_set_access_hint_on_compaction_start(
    rocksdb_options_t* opt, int v) {
  switch(v) {
    case 0:
      opt->rep.access_hint_on_compaction_start =
          ROCKSDB_NAMESPACE::Options::NONE;
      break;
    case 1:
      opt->rep.access_hint_on_compaction_start =
          ROCKSDB_NAMESPACE::Options::NORMAL;
      break;
    case 2:
      opt->rep.access_hint_on_compaction_start =
          ROCKSDB_NAMESPACE::Options::SEQUENTIAL;
      break;
    case 3:
      opt->rep.access_hint_on_compaction_start =
          ROCKSDB_NAMESPACE::Options::WILLNEED;
      break;
  }
}
```

Pull Request resolved: https://github.com/facebook/rocksdb/pull/8065

Reviewed By: riversand963

Differential Revision: D27102892

Pulled By: zhichao-cao

fbshipit-source-id: ad1d20d192712878e61597311ba75b55df0066d7

db/c.cc

@@ -2959,6 +2959,8 @@ void rocksdb_options_set_access_hint_on_compaction_start(
       opt->rep.access_hint_on_compaction_start =
           ROCKSDB_NAMESPACE::Options::WILLNEED;
       break;
+    default:
+      assert(0);
   }
 }