Fix propertyset's builtin attribute

git-svn-id: https://svn.apache.org/repos/asf/ant/core/trunk@277032 13f79535-47bb-0310-9956-ffa450edef68

src/main/org/apache/tools/ant/types/PropertySet.java

@@ -76,7 +76,7 @@ public class PropertySet extends DataType {
 
         public void setBuiltin(BuiltinPropertySetName b) {
             String built_in = b.getValue();
-            assertValid("builtin", builtin);
+            assertValid("builtin", built_in);
             this.builtin = built_in;
         }