FA M3 Red Dawn: Fix bug with 1st mission completion due to external factories

[maudlin27]

Following FAF changes introducing external factory units, these get included as part of the objective to be destroyed, but the objective doesn't complete when the parent unit is destroyed (presumably the OnKilled callback doesnt trigger for the external factory when it is removed due to the parent dying) Fix is to just exclude external factories from the list of units added to the objective.

[Garanas]

I thought I fixed this, the callbacks should trigger with the latest hot fix