Fix bindings of narrowing functions.

* latex.el (LaTeX-common-initialization):

* tex.el (VirTeX-common-initialization): Move
`LaTeX-narrow-to-environment' and `TeX-narrow-to-group' bindings
to the AUCTeX maps because `narrow-map' is not defined in GNU
Emacs < 22.2 and XEmacs.  Reported by Giacomo Boffi.