[usockets] Fix build issue with feature ssl

[PhoebeHui]

Fixes #14933

sni* are in sni_tree.cpp, which not built in vcpkg, including this file, and it requires c++17 or later. See https://github.com/uNetworking/uSockets/blob/master/Makefile#L43

All features test pass with x64-windows and x64-windows-static

Failures:

openssl.c.obj : error LNK2019: unresolved external symbol _sni_new referenced in function _us_internal_create_ssl_socket_context
openssl.c.obj : error LNK2019: unresolved external symbol _sni_free referenced in function _us_internal_ssl_socket_context_free
openssl.c.obj : error LNK2019: unresolved external symbol _sni_add referenced in function _us_internal_ssl_socket_context_add_server_name
openssl.c.obj : error LNK2019: unresolved external symbol _sni_remove referenced in function _us_internal_ssl_socket_context_remove_server_name
openssl.c.obj : error LNK2019: unresolved external symbol _sni_find referenced in function _resolve_context
uSockets.dll : fatal error LNK1120: 5 unresolved externals

[BillyONeal]

Thanks for your help!