add missing truffle boundary

Author: timfel

graalpython/com.oracle.graal.python/src/com/oracle/graal/python/builtins/modules/BuiltinFunctions.java

@@ -1303,6 +1303,7 @@ public abstract static class BuiltinNode extends PythonUnaryBuiltinNode {
         @Child GetItemNode getNameNode = GetItemNode.create();
 
         @Specialization
+        @TruffleBoundary
         public Object doIt(PFunction func) {
             /*
              * (tfel): To be compatible with CPython, builtin module functions must be bound to