upstream: loading security keys into ssh-agent used the extension

constraint "sk-provider@openssh.com", not "sk@openssh.com"; spotted by Ron
Frederick

OpenBSD-Commit-ID: dbfba09edbe023abadd5f59c1492df9073b0e51d
diff --git a/PROTOCOL.u2f b/PROTOCOL.u2f
index 375d73b..823f536 100644
--- a/PROTOCOL.u2f
+++ b/PROTOCOL.u2f
@@ -205,7 +205,7 @@
 library for the key. The format of this constraint extension would be:
 
 	byte		SSH_AGENT_CONSTRAIN_EXTENSION
-	string		sk@openssh.com
+	string		sk-provider@openssh.com
 	string		middleware path
 
 This constraint-based approach does not present any compatibility