lib: rsa: Fix PKCS11 URI if one is not given in `keydir`

If `keydir` is not present, we need to build a PKCS11 URI
from just the key name. In this case, we *do* need 'pkcs11:'

Fixes: ece85cc0 rsa: use pkcs11 uri as defined in rfc7512

Signed-off-by: Csókás Bence <csokas.bence@prolan.hu>