MAINTAINERS: exec: Add auxvec.h UAPI

Changes to auxiliary vectors would be best reviewed by the exec/binfmt
maintainers.

Link: https://lore.kernel.org/r/20241031220833.work.539-kees@kernel.org


Reviewed-by: Eric W. Biederman <ebiederm@xmission.com>
Signed-off-by: Kees Cook <kees@kernel.org>