Implemented proper functionality of SE_Screech

If you have a buff with SE_Screech with value of 1
it will block any other buff with SE_Screen that has
a value of -1, giving you an immunity message.
Example: 1383 Screech and 2785 Screech Immunity

zone/common.h

@@ -346,6 +346,7 @@ struct StatBonuses {
 	uint16	AbsorbMagicAtt[2];					// 0 = magic rune value 1 = buff slot
 	uint16	MeleeRune[2];						// 0 = rune value 1 = buff slot
 	bool	NegateIfCombat;						// Bool Drop buff if cast or melee
+	int8	Screech;							// -1 = Will be blocked if another Screech is +(1)
 
 	// AAs
 	int8	Packrat;							//weight reduction for items, 1 point = 10%