http_server: no "[[fallthrough]]" for GCC older than 7
http_server: no "[[fallthrough]]" for GCC older than 7

In older GCC versions, this causes error messages.

CM4all/beng-proxy,CM4all/beng-proxy,CM4all/beng-proxy,CM4all/beng-proxy,CM4all/beng-proxy,CM4all/beng-proxy