Fix attribute check for gnutls

As of version 3.6.6 gnutls checks to see if we have the CKA_SIGN and
CKA_DECRYPT attributes set for PSS signing and OAEP decryption keys.
These are set in the cache but not processed by our attribute handling
code, so add them.

Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>