vendor: update buildkit to f238f1e

Signed-off-by: Tonis Tiigi <tonistiigi@gmail.com>

vendor/github.com/containerd/containerd/plugin/plugin.go

@@ -20,6 +20,7 @@ import (
 	"fmt"
 	"sync"
 
+	"github.com/containerd/ttrpc"
 	"github.com/pkg/errors"
 	"google.golang.org/grpc"
 )
@@ -123,6 +124,16 @@ type Service interface {
 	Register(*grpc.Server) error
 }
 
+// TTRPCService allows TTRPC services to be registered with the underlying server
+type TTRPCService interface {
+	RegisterTTRPC(*ttrpc.Server) error
+}
+
+// TCPService allows GRPC services to be registered with the underlying tcp server
+type TCPService interface {
+	RegisterTCP(*grpc.Server) error
+}
+
 var register = struct {
 	sync.RWMutex
 	r []*Registration