libs: libwebsockets: add latest released version

This includes a patch for cyassl building that has been committed
upstream, but not released.

There's no SPDX license identifier for LGPL + static linking exception,
so I've used "LGPL-2.1+exception" as suggested at
http://lwn.net/Articles/378329/

See also: https://github.com/warmcat/libwebsockets/issues/170

Signed-off-by: Karl Palsson <karlp@remake.is>
Merge pull request #671 from remakeelectric/libwebsockets

libs: libwebsockets: add latest released version
mpd: Update to 0.18.20

Signed-off-by: Ted Hess <thess@kitschensync.net>