tka: use ts_omit_tailnetlock in another spot, for ed25519consensus

I noticed this while modularizing clientupdate. With this in first,
moving clientupdate to be modular removes a bunch more stuff from
the minimal build + tsnet.

Updates #17115

Change-Id: I44bd055fca65808633fd3a848b0bbc09b00ad4fa
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

tka/verify_disabled.go

@@ -0,0 +1,18 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+//go:build ts_omit_tailnetlock
+
+package tka
+
+import (
+	"errors"
+
+	"tailscale.com/types/tkatype"
+)
+
+// signatureVerify returns a nil error if the signature is valid over the
+// provided AUM BLAKE2s digest, using the given key.
+func signatureVerify(s *tkatype.Signature, aumDigest tkatype.AUMSigHash, key Key) error {
+	return errors.New("tailnetlock disabled in build")
+}