add required internal TLS API's for dummy and schannel

Signed-off-by: Steffen Jaeckel <jaeckel-floss@eyet-services.de>

src/tls_schannel.c

@@ -222,6 +222,13 @@ void tls_free(tls_t *tls)
     return;
 }
 
+xmpp_tlscert_t *tls_peer_cert(xmpp_conn_t *conn)
+{
+    /* always fail */
+    xmpp_error(conn->ctx, "tls", "tls_peer_cert() not implemented");
+    return NULL;
+}
+
 int tls_set_credentials(tls_t *tls, const char *cafilename)
 {
     UNUSED(tls);