When using "Make Links"->"Materials" regular materials could be linked onto grease pencil objects. This caused a number of issues. The fix changes the `allow_make_links_data` function to make sure that if one object is of type `OB_GPENCIL`, the other has to be aswell. Reviewed By: antoniov Maniphest Tasks: T84420 Differential Revision: https://developer.blender.org/D10014