Call init-loader-show-log only when init-loader-show-log-after-init is t

[README](https://github.com/emacs-jp/init-loader#init-loader-show-log-after-init--boolean-default-t) describes about `init-loader-show-log-after-init` that "Show log message after initializing if this value is `t`" but current implementation after ccb72c9f62e1271da1518133c6937d85 is the following:

* `(setq init-loader-show-log-after-init t)`
  * `init-loader-show-log` is called
* `(setq init-loader-show-log-after-init nil)`
  * `init-loader-show-log` is called