vendor: update buildkit to 539be170

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

vendor/golang.org/x/sys/plan9/pwd_go15_plan9.go

@@ -2,6 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+//go:build go1.5
 // +build go1.5
 
 package plan9

vendor/golang.org/x/sys/plan9/pwd_plan9.go

@@ -2,6 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+//go:build !go1.5
 // +build !go1.5
 
 package plan9

vendor/golang.org/x/sys/plan9/race.go

@@ -2,6 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+//go:build plan9 && race
 // +build plan9,race
 
 package plan9

vendor/golang.org/x/sys/plan9/race0.go

@@ -2,6 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+//go:build plan9 && !race
 // +build plan9,!race
 
 package plan9

vendor/golang.org/x/sys/plan9/str.go

@@ -2,6 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+//go:build plan9
 // +build plan9
 
 package plan9

vendor/golang.org/x/sys/plan9/syscall.go

@@ -2,6 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+//go:build plan9
 // +build plan9
 
 // Package plan9 contains an interface to the low-level operating system

vendor/golang.org/x/sys/plan9/zsyscall_plan9_386.go

@@ -1,6 +1,7 @@
 // go run mksyscall.go -l32 -plan9 -tags plan9,386 syscall_plan9.go
 // Code generated by the command above; see README.md. DO NOT EDIT.
 
+//go:build plan9 && 386
 // +build plan9,386
 
 package plan9

vendor/golang.org/x/sys/plan9/zsyscall_plan9_amd64.go

@@ -1,6 +1,7 @@
 // go run mksyscall.go -l32 -plan9 -tags plan9,amd64 syscall_plan9.go
 // Code generated by the command above; see README.md. DO NOT EDIT.
 
+//go:build plan9 && amd64
 // +build plan9,amd64
 
 package plan9

vendor/golang.org/x/sys/plan9/zsyscall_plan9_arm.go

@@ -1,6 +1,7 @@
 // go run mksyscall.go -l32 -plan9 -tags plan9,arm syscall_plan9.go
 // Code generated by the command above; see README.md. DO NOT EDIT.
 
+//go:build plan9 && arm
 // +build plan9,arm
 
 package plan9