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

The change in commit 901386f646 (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 ")")`.

Auxiliary/cmake-mode.el

@@ -211,7 +211,7 @@ the indentation.  Otherwise it retains the same position on the line"
       "end"
       (or "function" "macro")
       (zero-or-more space)
-      "(" (zero-or-more (not ")")) ")"))
+      "(" (zero-or-more (not-char ")")) ")"))
 
 (defun cmake-beginning-of-defun ()
   "Move backward to the beginning of a CMake function or macro.