Added missing DB index

I have enabled the "slow queries" log in MySQL and found these queries:

```sql
SELECT e0_.content AS content0, e0_.field AS field1, e0_.locale AS locale2
FROM ext_translations e0_
WHERE e0_.foreign_key = 61 AND e0_.object_class = 'MyClass'
ORDER BY e0_.locale ASC;
```

The `EXPLAIN` showed me that no index was used, so I added the missing index in the mapping.