cmake-mode.el: Fix rx usage for Emacs 26

The change in commit 901386f6 (cmake-mode.el: Add navigation by
function and macro, 2020-12-11) introduced a regression.  Emacs 26 could
not load cmake-mode.el anymore.  Use `(not-char ")")`, not `(not ")")`.
4 jobs for Not-char--not-not
Status Name Job ID Coverage
  External
passed cdash

passed ghostflow-check-master

passed cdash

passed cdash