sys: add import comments.

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

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"