go.crypto/ssh: hide private forwardList methods

This was my mistake. I should have checked godoc before
submitting the previous CL.

R=agl
CC=golang-dev
https://golang.org/cl/6140051

ssh/client.go

@@ -301,7 +301,7 @@ func (c *ClientConn) handleChanOpen(msg *channelOpenMsg) {
 			c.sendConnectionFailed(msg.PeersId)
 			return
 		}
-		l, ok := c.forwardList.Lookup(laddr)
+		l, ok := c.forwardList.lookup(laddr)
 		if !ok {
 			fmt.Println("could not find forward list entry for", laddr)
 			// Section 7.2, implementations MUST reject suprious incoming