[ticket/12771] Bug in profilefield_base_migration when used in ext

The base migration class should create lang entry for the new CPF
but it is supposed to strip it of phpbb_ prefix. As extensions
CPFs do not use phpbb_ prefix it will not create a lang entry.
This is as simple solution as it can be done.

PHPBB3-12771
Wouldn't strpos be better than substr in the conditional?
```
$lang_name = (strpos($this->profilefield_name, 'phpbb_') === 0) ? strtoupper(substr($this->profilefield_name, 6)) : strtoupper($this->profilefield_name);
```