In BP_User_Query, allow 'include' params to be combined with the 'user_id' filter

This enhancement allows plugins to filter friend lists by passing an 'include'
parameter to BP_User_Query.

Fixes #4938

Props dontdream

git-svn-id: https://buddypress.svn.wordpress.org/trunk@6934 cdf35c40-ae34-48e0-9cc9-0c9da1808c22