fix(Core/Trainers): add new column ReqSpell to npc_trainer table (#9490)

Prevent learning gnomish and goblin engineering recipes at same time.
Fixes #5390

Co-authored-by: Skjalf <47818697+Nyeriah@users.noreply.github.com>

src/server/game/Entities/Creature/CreatureData.h

@@ -416,6 +416,7 @@ struct TrainerSpell
     uint32 reqSkillValue{0};
     uint32 reqLevel{0};
     uint32 learnedSpell[3];
+    uint32 reqSpell{0};
 
     // helpers
     [[nodiscard]] bool IsCastable() const { return learnedSpell[0] != spell; }