net: fix vet nit

net/fd_windows.go:121: syscall.WSABuf composite literal uses unkeyed fields

Change-Id: I91cbe38199d5b6828379a854d08f6ceaf687dd82
Reviewed-on: https://go-review.googlesource.com/32760
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/net/fd_windows.go

@@ -118,7 +118,7 @@ func (o *operation) InitBufs(buf *Buffers) {
 		if len(b) > 0 {
 			p = &b[0]
 		}
-		o.bufs = append(o.bufs, syscall.WSABuf{uint32(len(b)), p})
+		o.bufs = append(o.bufs, syscall.WSABuf{Len: uint32(len(b)), Buf: p})
 	}
 }