devel/websocketpp: fix asio namespaces unbreaking users

and unbreak multimedia/obs-studio.

This works by using standalone asio's namespace
instead of boost::asio's everywhere. (Boost includes asio,
and websocketpp can work with Boost and the standalone Asio,
pulling it into its own namespace wrapper.)

Tested by: tagattie@, mandree@

PR: 287916
(cherry picked from commit c3fe791f1a841cbe6c101bef256740e2e7d30218)