x/sys/unix: regenerate zsyscall_* files

Fixes build breakage due to missing syscall import.

LGTM=adg, r
R=r, adg
CC=golang-codereviews
https://golang.org/cl/162650044
diff --git a/unix/zsyscall_dragonfly_amd64.go b/unix/zsyscall_dragonfly_amd64.go
index 3a226ae..d75c671 100644
--- a/unix/zsyscall_dragonfly_amd64.go
+++ b/unix/zsyscall_dragonfly_amd64.go
@@ -3,7 +3,12 @@
 
 package unix
 
-import "unsafe"
+import (
+	"syscall"
+	"unsafe"
+)
+
+var _ syscall.Errno
 
 // THIS FILE IS GENERATED BY THE COMMAND AT THE TOP; DO NOT EDIT