server : fix the log message when using SSL (#23393)

When llama-server is started with SSL key and cert, the log says that it
listens on http instead of https. This patch fixes this.

tools/server/server-http.h

@@ -74,6 +74,7 @@ struct server_http_context {
     std::string path_prefix;
     std::string hostname;
     int port;
+    bool is_ssl = false;
 
     server_http_context();
     ~server_http_context();