Fix DragTarget `component` prop type

Summary: Previously due to a bug in React, component types were counted as component descriptors by this prop check; now they're not and this check fails. There's no type for a React component class, but component types are all functions so let's use that instead.

Test Plan: Crossed fingers.

Reviewers: joel

Reviewed By: joel

Subscribers: jessie

Differential Revision: http://phabricator.khanacademy.org/D11466