MAINTAINERS: Foolishly claim maintainership of string routines

Since the string API is tightly coupled with FORTIFY_SOURCE, I am
offering myself up as maintainer for it. Thankfully Andy is already a
reviewer and can keep me on the straight and narrow.

Acked-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Link: https://lore.kernel.org/r/20230712194625.never.252-kees@kernel.org


Signed-off-by: Kees Cook <keescook@chromium.org>