sys: add import comments.

Change-Id: I0ebbb4f2da41bea7871f893332f2e4847b1bfefa
Reviewed-on: https://go-review.googlesource.com/1241Reviewed-by: Andrew Gerrand <adg@golang.org>

plan9/syscall.go

@@ -19,7 +19,7 @@
 // These calls return err == nil to indicate success; otherwise
 // err represents an operating system error describing the failure and
 // holds a value of type syscall.ErrorString.
-package plan9
+package plan9 // import "golang.org/x/sys/plan9"
 
 import "unsafe"
 

unix/syscall.go

@@ -19,7 +19,7 @@
 // These calls return err == nil to indicate success; otherwise
 // err represents an operating system error describing the failure and
 // holds a value of type syscall.Errno.
-package unix
+package unix // import "golang.org/x/sys/unix"
 
 import "unsafe"
 

windows/syscall.go

@@ -19,7 +19,7 @@
 // These calls return err == nil to indicate success; otherwise
 // err represents an operating system error describing the failure and
 // holds a value of type syscall.Errno.
-package windows
+package windows // import "golang.org/x/sys/windows"
 
 import (
 	"syscall"