Fix fontification in Emacs27.

Some more breaking cc-mode changes. Completely halts all fontification.

Fix it by ignoring it... For now.

Author: josteink

csharp-mode.el

@@ -753,7 +753,8 @@ to work properly with code that includes attributes."
              ;; Match a char before the string starter to make
              ;; `c-skip-comments-and-strings' work correctly.
              (concat ".\\(" c-string-limit-regexp "\\)")
-             '((c-font-lock-invalid-string)))
+             '((when (fboundp 'c-font-lock-invalid-string)
+                 (c-font-lock-invalid-string))))
 
 
            ;; Fontify keyword constants.