Add visibility to function

[mricaud]

Even if these XSLT doesn't use packages it might be a good idea to clarify visibility of each component functions
https://www.w3.org/TR/xslt-30/#visibility
(as suggested by @martin-honnen)
It's alway usefull to have this information, even if it's not used with package mechanism. It will be ready for the day it will use packages.