Fix go-libutp import when CGO is disabled

socket.go

@@ -5,7 +5,6 @@ import (
 	"net"
 	"strconv"
 
-	utp "github.com/anacrolix/go-libutp"
 	"github.com/anacrolix/missinggo/perf"
 	"github.com/anacrolix/missinggo/v2"
 	"github.com/pkg/errors"
@@ -101,7 +100,8 @@ func listenAllRetry(nahs []networkAndHost, port int, f firewallCallback) (ss []s
 	return
 }
 
-type firewallCallback = utp.FirewallCallback
+// This isn't aliased from go-libutp since that assumes CGO.
+type firewallCallback func(net.Addr) bool
 
 func listenUtp(network, addr string, fc firewallCallback) (socket, error) {
 	us, err := NewUtpSocket(network, addr, fc)

utp_libutp.go

@@ -16,7 +16,7 @@ func NewUtpSocket(network, addr string, fc firewallCallback) (utpSocket, error)
 		return s, err
 	}
 	if fc != nil {
-		s.SetSyncFirewallCallback(fc)
+		s.SetSyncFirewallCallback(utp.FirewallCallback(fc))
 	}
 	return s, err
 }