os: fix windows build

R=brainman
CC=golang-dev
https://golang.org/cl/4308047

src/pkg/os/file_windows.go

diff --git a/src/pkg/os/file_windows.go b/src/pkg/os/file_windows.go
index 16dd4b6..862baf6 100644
--- a/src/pkg/os/file_windows.go
+++ b/src/pkg/os/file_windows.go
@@ -168,7 +168,7 @@
 // Truncate changes the size of the named file.
 // If the file is a symbolic link, it changes the size of the link's target.
 func Truncate(name string, size int64) Error {
-	f, e := Open(name, O_WRONLY|O_CREATE, 0666)
+	f, e := OpenFile(name, O_WRONLY|O_CREATE, 0666)
 	if e != nil {
 		return e
 	}