kdc: add audit plugin API to windc API

Allow the windc plugin to also implement an audit callback. As part of this
change, both the HDB and windc audit function signatures are changed to return
void.

tests/plugin/windc.c

@@ -116,6 +116,13 @@ finalize_reply(void *ctx, astgs_request_t r)
     return 0;
 }
 
+static krb5_error_code KRB5_CALLCONV
+audit(void *ctx, astgs_request_t r)
+{
+    logit("audit", r);
+    return 0;
+}
+
 static krb5plugin_windc_ftable windc = {
     KRB5_WINDC_PLUGING_MINOR,
     windc_init,
@@ -124,7 +131,8 @@ static krb5plugin_windc_ftable windc = {
     pac_verify,
     client_access,
     NULL, /* referral_policy */
-    finalize_reply
+    finalize_reply,
+    audit
 };
 
 static const krb5plugin_windc_ftable *const windc_plugins[] = {