MAINTAINERS: Add ubsan.h to the UBSAN section

The "ubsan.h" file was missed in the creation of the UBSAN section. Add
it.

Link: https://lore.kernel.org/r/20240424162722.work.576-kees@kernel.org
Acked-by: Marco Elver <elver@google.com>
Signed-off-by: Kees Cook <keescook@chromium.org>