Scripts/Ulduar: Fixed a clang6 warning

(cherry picked from commit 80a65744bc)

src/server/scripts/Northrend/Ulduar/Ulduar/boss_flame_leviathan.cpp

@@ -1631,6 +1631,8 @@ class FlameLeviathanPursuedTargetSelector
     };
 
     public:
+        explicit FlameLeviathanPursuedTargetSelector(Unit* unit) { };
+
         bool operator()(WorldObject* target) const
         {
             //! No players, only vehicles. Pursue is never cast on players.