keys: Add Jarkko Sakkinen as co-maintainer

To address a major procedural concern on Linus's part the keyrings needs
a co-maintainer.

Suggested-by: David Howells <dhowells@redhat.com>
Signed-off-by: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>
Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>