MAINTAINERS: add "exec & binfmt" section with myself and Eric

I'd like more continuity of review for the exec and binfmt (and ELF)
stuff.  Eric and I have been the most active lately, so list us as
reviewers.

Link: https://lkml.kernel.org/r/20211006180200.1178142-1-keescook@chromium.org


Signed-off-by: Kees Cook <keescook@chromium.org>
Cc: Eric Biederman <ebiederm@xmission.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>