alias lws_plat_service_tsi to lws_service_tsi in public api

alias lws_plat_service_tsi to lws_service_tsi in public api

Via Dosvald

lws_service_tsi() which has been around a while actually just
calls through to lws_plat_service_tsi(), meaning there is no
need to expose both apis.

Rename the internal lws_plat_service_tsi() to _lws_plat_service_tsi()
and replace the api export with a #define to lws_service_tsi for
compatibility's sake.