KEYS: Store public key algo ID in public_key_signature struct

Store public key algorithm ID in public_key_signature struct for reference
purposes.  This allows a public_key_signature struct to be embedded in
struct x509_certificate and other places more easily.

Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Josh Boyer <jwboyer@redhat.com>