cmake-mode.el: Fix "unescaped character literals" warning

Emacs 27.0 warns

    Loading ‘cmake-mode’: unescaped character literals `?(', `?)' detected!

during byte-compilation of cmake-mode.el The new warning was added in
emacs commit c2bbdc3316 (Warn about missing backslashes during load).

Add backslashes to escape the literals.

Auxiliary/cmake-mode.el

@@ -55,7 +55,7 @@ set the path with these commands:
       (* (or (not (any space "()#\\\n")) (and ?\\ nonl)))))
 (defconst cmake-regex-token
   (rx-to-string `(group (or (regexp ,cmake-regex-comment)
-                            ?( ?)
+                            ?\( ?\)
                             (regexp ,cmake-regex-argument-unquoted)
                             (regexp ,cmake-regex-argument-quoted)))))
 (defconst cmake-regex-indented