Fix incorrect variable name

git-svn-id: https://buddypress.svn.wordpress.org/trunk@7814 cdf35c40-ae34-48e0-9cc9-0c9da1808c22
Use API function instead of direct query in BP_Activity_Activity::append_user_fullnames()

This allows for better caching.

git-svn-id: https://buddypress.svn.wordpress.org/trunk@7815 cdf35c40-ae34-48e0-9cc9-0c9da1808c22