cmake: Fix DetectChar warning about escaped backslash

Warning of highlighting indexer:

katehighlightingindexer(8268)/(default) {anonymous}::checkSingleChars: "/home/dh/kde/kf5/src/frameworks/syntax-highlighting/data/syntax/cmake.xml" line 3497 'char' must contain exactly one char: "\\\\"
katehighlightingindexer(8268)/(default) {anonymous}::checkSingleChars: "/home/dh/kde/kf5/src/frameworks/syntax-highlighting/data/syntax/cmake.xml" line 3498 'char' must contain exactly one char: "\\\\"

CCMAIL: Alex Turbov <i.zaufi@gmail.com>