mirrors: add repo.extreme-ix.org back to the tier 2 mirrors

it's still syncing, so might as well keep it

Author: lazka

web/dev/mirrors.md

@@ -63,6 +63,7 @@ support (\*), HTTPS support, synced at least once per day from the primary serve
 | mirror.iscas.ac.cn | [HTTPS](https://mirror.iscas.ac.cn/msys2/) | |
 | mirrors.cloud.tencent.com | [HTTPS](https://mirrors.cloud.tencent.com/msys2/) | <petzhou@tencent.com> |
 | download.nus.edu.sg | [HTTPS](https://download.nus.edu.sg/mirror/msys2/) | <download@nus.edu.sg> |
+| repo.extreme-ix.org | [HTTPS](https://repo.extreme-ix.org/msys2/) \| [RSYNC](rsync://repo.extreme-ix.org/msys2/) | <sysadmin@x3me.net> |
 
 ## Adding a New Mirror