(Partly) Fix PetraAI building a lot of siege and not using it.
Following r25584 / rP25584.
The used classes for adding siege were wrong.
Reported in https://wildfiregames.com/forum/topic/45178-a25-svn-version-25810-not-playable/.
Differential revision: https://code.wildfiregames.com/D4184
Refs. #6250 by (partly) fixing the siege case.
Reviewed by: @Angen