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