Enable keyword arguments for keyword_rename unit test

This is required for the recently added Python test checking that an
argument clashing with a Python keyword is also renamed when using
keyword arguments and fixes this test failure when using -builtin Python
option.

Examples/test-suite/keyword_rename.i

@@ -4,6 +4,8 @@
 
 %module keyword_rename
 
+%feature("kwargs");
+
 #pragma SWIG nowarn=SWIGWARN_PARSE_KEYWORD
 
 %inline %{