os,syscall: fix windows build

make syscall.ProcAttr.Files be []uintptr

all.bash passes on Linux.
things seem to compile on GOOS={darwin,windows}

R=golang-dev, mattn.jp, alex.brainman, rsc
CC=golang-dev
https://golang.org/cl/5653055
diff --git a/src/pkg/syscall/exec_windows.go b/src/pkg/syscall/exec_windows.go
index 6cb25a7..4dc4d05 100644
--- a/src/pkg/syscall/exec_windows.go
+++ b/src/pkg/syscall/exec_windows.go
@@ -220,7 +220,7 @@
 type ProcAttr struct {
 	Dir   string
 	Env   []string
-	Files []Handle
+	Files []uintptr
 	Sys   *SysProcAttr
 }