websockets: fix compatibility with older lws versions (#260)

In 1.3, 1.4 and 1.5, the function was "libwebsockets_get_protocol" not
"libwebsocket_get_protocol"  While the #define name doesn't matter on
newer libwebsockets, where it redirects to lws_get_protocol, the naming
is critical for older versions.

Fixes: 477cd3e3

 (Fix missing context->listener for websocket client)

Signed-off-by: Karl Palsson <karlp@etactica.com>