libwebsockets: bump to version 2.1.1

Mosquitto combined with version 2.1.0 of libwebsockets yields a failing
system. Version 2.1.1 restores the websocket interface again.

Signed-off-by: Kurt Van Dijck <dev.kurt@vandijck-laurijssen.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>