Use correct `$exclude` variable (rather than `$r`) in `BP_Core_User::get_users_by_letter()`. Fixes ability to query for users by `letter` and also exclude certain users.

Querying for users by letter has not been used in BuddyPress's template parts since 1.2, but is still actively supported for other plugins and themes to use.



git-svn-id: https://buddypress.svn.wordpress.org/trunk@8558 cdf35c40-ae34-48e0-9cc9-0c9da1808c22