Fixes fortified defender headbutt (#5004)

# About the pull request

Fixes #5003 

# Explain why it's good for the game


# Testing Photographs and Procedure
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<summary>Screenshots & Videos</summary>

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# Changelog
:cl:
fix: Fixes defender headbutt bug while fortified.
/:cl:

code/modules/mob/living/carbon/xenomorph/abilities/defender/defender_powers.dm

@@ -51,7 +51,7 @@
 	if(!check_and_use_plasma_owner())
 		return
 
-	if(fendy.fortify && !fendy.mutation_type == DEFENDER_STEELCREST)
+	if(fendy.fortify && !(fendy.mutation_type == DEFENDER_STEELCREST))
 		to_chat(fendy, SPAN_XENOWARNING("You cannot use headbutt while fortified."))
 		return