tests/server: replace strerror with sstrerror in socksd

To follow the `errno` -> `SOCKERRNO` update.

Missed from the previous commit.

Follow-up to adcfd4fb3e #16553

tests/server/socksd.c

@@ -744,7 +744,7 @@ static bool incoming(curl_socket_t listenfd)
 
     if(rc < 0) {
       logmsg("select() failed with error: (%d) %s",
-             error, strerror(error));
+             error, sstrerror(error));
       return FALSE;
     }