Scripts/Zul'farrak: Fixed summoning Gahz'rilla

Fixes #5782 regression

(cherry picked from commit cec05ff07d1751649b6eecff07f23f52ca5443cd)

src/server/scripts/Kalimdor/ZulFarrak/instance_zulfarrak.cpp

@@ -111,6 +111,7 @@ public:
         instance_zulfarrak_InstanceMapScript(Map* map) : InstanceScript(map)
         {
             SetHeaders(DataHeader);
+            GahzRillaEncounter = NOT_STARTED;
             PyramidPhase = 0;
             major_wave_Timer = 0;
             minor_wave_Timer = 0;