Scripts/BlackwingLair: add missing one-time validation in Nefarian's SAY_XHEALTH

(cherry picked from commit 542b0fbf4b122d0204684b20d0abdef23137c773)

src/server/scripts/EasternKingdoms/BlackrockMountain/BlackwingLair/boss_nefarian.cpp

@@ -435,6 +435,7 @@ struct boss_nefarian : public BossAI
         _phase3 = false;
         _despawn = false;
         _despawnTimer = 30000;
+        _sayLowHealth = false;
     }
 
     void Reset() override
@@ -501,8 +502,11 @@ struct boss_nefarian : public BossAI
             Talk(SAY_RAISE_SKELETONS);
         }
 
-        if (me->HealthBelowPctDamaged(5, damage))
+        if (!_sayLowHealth && me->HealthBelowPctDamaged(5, damage))
+        {
+            _sayLowHealth = true;
             Talk(SAY_XHEALTH);
+        }
     }
 
     void UpdateAI(uint32 diff) override
@@ -614,6 +618,7 @@ private:
     bool _despawn;
     uint32 _despawnTimer;
     bool _phase3;
+    bool _sayLowHealth;
 };
 
 void AddSC_boss_nefarian()