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: ece85cc020 rsa: use pkcs11 uri as defined in rfc7512

Signed-off-by: Csókás Bence <csokas.bence@prolan.hu>
diff --git a/lib/rsa/rsa-sign.c b/lib/rsa/rsa-sign.c
index 858ad92..fd587d8 100644
--- a/lib/rsa/rsa-sign.c
+++ b/lib/rsa/rsa-sign.c
@@ -124,7 +124,7 @@
 					 keydir, name);
 		else
 			snprintf(key_id, sizeof(key_id),
-				 "object=%s;type=public",
+				 "pkcs11:object=%s;type=public",
 				 name);
 	} else if (engine_id) {
 		if (keydir)
@@ -246,7 +246,7 @@
 					 keydir, name);
 		else
 			snprintf(key_id, sizeof(key_id),
-				 "object=%s;type=private",
+				 "pkcs11:object=%s;type=private",
 				 name);
 	} else if (engine_id) {
 		if (keydir && name)