Add missing '@TruffleBoundary'.

Author: fangerer

graalpython/com.oracle.graal.python/src/com/oracle/graal/python/builtins/objects/type/TypeBuiltins.java

@@ -457,6 +457,7 @@ static abstract class SubclassesNode extends PythonUnaryBuiltinNode {
         @Child private IsSubtypeNode isSubtypeNode = IsSubtypeNode.create();
 
         @Specialization
+        @TruffleBoundary
         PList getSubclasses(PythonClass cls) {
             // TODO: missing: keep track of subclasses
             return factory().createList(cls.getSubClasses().toArray());