wgengine/magicsock: fix watchdog timeout on Close when IPv6 not available

The blockForeverConn was only using its sync.Cond one side. Looks like it
was just forgotten.

Fixes #3671

Change-Id: I4ed0191982cdd0bfd451f133139428a4fa48238c
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

wgengine/magicsock/magicsock.go

@@ -3121,6 +3121,7 @@ func (c *blockForeverConn) Close() error {
 		return net.ErrClosed
 	}
 	c.closed = true
+	c.cond.Broadcast()
 	return nil
 }