MAINTAINERS: Add UBSAN section

The kernel hardening efforts have continued to depend more and more
heavily on UBSAN, so make an actual MAINTAINERS entry for it.

Cc: Andrey Ryabinin <ryabinin.a.a@gmail.com>
Acked-by: Andrey Konovalov <andreyknvl@gmail.com>
Acked-by: Marco Elver <elver@google.com>
Signed-off-by: Kees Cook <keescook@chromium.org>