Do not override font-lock for case/continue/break labels

This prevents us from displaying words following "case", "continue",
and "break" in unexpected faces inside comments.

Consider this (meaningless) comment haiku:

    // continue to move
    // I will break all of the things
    // in case of failure

Before this commit is applied, "to", "all", and "of" are displayed with unexpected coloration.

Based on a little experimentation and [this page](http://www.gnu.org/software/emacs/manual/html_node/elisp/Search_002dbased-Fontification.html#Search_002dbased-Fontification), I believe this patch fixes the issue, but I might be mistaken.