Mark SPK's signatures output as required.

AuthenticationExtensionsSupplementalPubKeysOutputs.signatures should
always be present in the client extension outputs since it's the only
field and SPK makes no sense without the signatures.

index.bs

@@ -7253,7 +7253,7 @@ The weight that [=[RPS]=] give to the presence of a signature from a supplementa
 :: A sequence of {{ArrayBuffer}}s containing the signatures returned as the [=unsigned extension output=].
     <xmp class="idl">
     dictionary AuthenticationExtensionsSupplementalPubKeysOutputs {
-        sequence<ArrayBuffer> signatures;
+        required sequence<ArrayBuffer> signatures;
     };
 
     partial dictionary AuthenticationExtensionsClientOutputs {