krb5: Add an optional encrypt_iov function to encryption types

Add a encrypt_iov function pointer to all of our encryption types
which can be used to implement an iovec based encryption routine.

Modify krb5_encrypt_iov so that it calls the iovec based routine
if it is available.

lib/krb5/crypto-null.c

@@ -97,6 +97,7 @@ struct _krb5_encryption_type _krb5_enctype_null = {
     NULL,
     F_DISABLED,
     NULL_encrypt,
+    NULL,
     0,
     NULL
 };