added an explicit require for site-gentoo when it's available

I have disabled site-gentoo being loaded automatically on start by
editing site-start. Now I explicitly require it when it's available in
this config, but it can be omitted or toggled in other configs (a
minimal config for example).

init.el

@@ -1,3 +1,6 @@
+(when (locate-library "site-gentoo")
+  (require 'site-gentoo))
+
 (setq auth-source-save-behavior nil
       auto-save-default nil
       change-major-mode-with-file-name nil