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

This commit addresses the literal by adding a backslash to escaped the quote.

(The new warning was added in http://git.savannah.gnu.org/cgit/emacs.git/commit/?id=c2bbdc3316487e34eba1470dd059c0c290431e00)

Topic-rename: emacs-fix-unescaped-character-literals