Revert removal of roubert@google.com from OWNERS.

This partially reverts commit a903cc7, the bug that required this entry
to be removed then has been fixed now:

https://gerrit.googlesource.com/plugins/find-owners/+/7a4ec6a

Bug: 79676796
Test: repo upload
Change-Id: Ideda071d5262bee8e4376f292e4b1fb9db2c37d9
diff --git a/OWNERS b/OWNERS
index ae18de7..963d2c2 100644
--- a/OWNERS
+++ b/OWNERS
@@ -1,5 +1,6 @@
 icu-team+reviews@google.com
 mscherer@google.com
+roubert@google.com
 
 android-libcore-team+review@google.com
 vichang@google.com