KEYS: Make the X.509 and PKCS7 parsers supply the sig encoding type [ver #2]

Make the X.509 and PKCS7 parsers fill in the signature encoding type field
recently added to the public_key_signature struct.

Signed-off-by: David Howells <dhowells@redhat.com>
Tested-by: Marcel Holtmann <marcel@holtmann.org>
Reviewed-by: Marcel Holtmann <marcel@holtmann.org>
Reviewed-by: Denis Kenzior <denkenz@gmail.com>
Tested-by: Denis Kenzior <denkenz@gmail.com>
Signed-off-by: James Morris <james.morris@microsoft.com>