usbip: gate serverImportLease so unsupported-target builds compile

control_protocol.go was ungated but referenced ExportLeaseIdentity
from host.go (linux || (darwin && cgo)), breaking the stub-register
contract on Windows and darwin && !cgo. Move serverImportLease and
importLeaseTTL into export_ledger.go alongside their only consumers.

service/usbip/control_protocol.go

@@ -6,7 +6,6 @@ import (
 	"io"
 	"slices"
 	"strings"
-	"time"
 
 	E "github.com/sagernet/sing/common/exceptions"
 )
@@ -47,8 +46,6 @@ const (
 	leaseErrorBadRequest  = "bad_request"
 	leaseErrorUnavailable = "unavailable"
 	leaseErrorBusy        = "busy"
-
-	importLeaseTTL = 10 * time.Second
 )
 
 var controlPreface = [controlPrefaceSize]byte{'S', 'B', 'U', 'S', 'B', 'I', 'P', '1'}
@@ -121,16 +118,6 @@ type controlLeaseResponse struct {
 	ErrorMessage string `json:"error_message,omitempty"`
 }
 
-type serverImportLease struct {
-	ID           uint64
-	SubscriberID uint64
-	BusID        string
-	ClientNonce  uint64
-	Generation   uint64
-	Identity     ExportLeaseIdentity
-	Expires      time.Time
-}
-
 // controlReader reuses its payload scratch across successive reads on a
 // single connection. The returned payload is only valid until the next call.
 type controlReader struct {

service/usbip/export_ledger.go

@@ -45,7 +45,20 @@ type exportSubscriber struct {
 	send         chan controlMessage
 }
 
-const controlSubscriberSendBuffer = 16
+type serverImportLease struct {
+	ID           uint64
+	SubscriberID uint64
+	BusID        string
+	ClientNonce  uint64
+	Generation   uint64
+	Identity     ExportLeaseIdentity
+	Expires      time.Time
+}
+
+const (
+	controlSubscriberSendBuffer = 16
+	importLeaseTTL              = 10 * time.Second
+)
 
 func newExportLedger(logger log.ContextLogger, ttl time.Duration, now func() time.Time) *exportLedger {
 	if now == nil {