GDExtension: Fix method binds not saying if they are varargs

(cherry picked from commit 2599df3b8ae5aa6b718feadefb39cab037927f3e)

core/extension/gdextension.cpp

@@ -296,7 +296,7 @@ public:
 	}
 
 	virtual bool is_vararg() const override {
-		return false;
+		return vararg;
 	}
 
 #ifdef TOOLS_ENABLED