Fix a missing method name.

I just stumbled upon a small error when trying to nuke my Firebird database with South:

```
$ manage.py migrate SouthRest zero

Running migrations for SouthRest:
 - Migrating backwards to zero state.
 < SouthRest:0004_hash_names
 < SouthRest:0003_auto__add_field_knight_hash
 < SouthRest:0002_auto__add_field_knight_data
 < SouthRest:0001_initial
Error in migration: SouthRest:0001_initial
AttributeError: 'DatabaseOperations' object has no attribute 'get_generator_name'
```

This small change fixes it. :-)