[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
move strtoupper() to here instead of on line 82