Cannot filter OG title in Views

When I try to add a filter to my View to filter by the group member's title (e.g., I want a listing of all users who are the group President), instead of the typical string filtering form, I don't get any inputs. I'd like to have a form which allows me to select the typical string operators - contains, is equal to, etc. - and a string on which to filter, e.g. "President" in this case.

This patch adds that filter, using the generic Views string filter handler. I've tested it in my own case and it seems to work correctly.