scripts/kernel-doc: Escape all literal braces in regexes

Commit 701b3a3c ("PATCH scripts/kernel-doc") fixed the two
instances of literal braces that Perl 5.28 warns about, but there are
still more than it doesn't warn about.

Escape all left braces that are treated as literal characters.  Also
escape literal right braces, for consistency and to avoid confusing
bracket-matching in text editors.

Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>