Minimal fix to build failure from #31211.

This one-line fix is already in main, from #31862. but that PR relies on other refactor PRs, and
all are too broad to apply to the patch release branch.

Signed-off-by: Scott Rigby <scott@r6by.com>

pkg/registry/registry_test.go

@@ -129,6 +129,7 @@ func setup(suite *TestRegistry, tlsEnabled, insecure bool, auth string) {
 
 	// Registry config
 	config := &configuration.Configuration{}
+	lnCfg := net.ListenConfig{}
 	ln, err := net.Listen("tcp", "127.0.0.1:0")
 	suite.Nil(err, "no error finding free port for test registry")
 	defer func() { _ = ln.Close() }()