build: not enabling service in install_systemd

[unlsycn]

Enabling service should be a post-installation behavior and is best performed manually by user. Invoke systemctl at installation will break the flow of package manager on environments without systemd as the service file is usually installed in such systems as well.

[unlsycn]

我在给graftcp打包，通常的行为是无论是否安装了systemd的系统，都可以安装systemd service文件。但是如果在安装时调用systemctl，就必须要判断系统是否使用systemd了。 这听上去问题不是很大，但是包管理器在安装一个包时自动启用了systemd的服务就是比较「僭越」的行为了，一般这种操作应该由用户手动完成。