Ever since [0], ReportList is not actually used in any DNA structs.
That makes sense, since reports are conceptually only necessary
at runtime. Move the definition of the struct to BKE_report.hh, and
fix a bunch of include errors where types were previously available
transitively. Besides adding some clarity, theoretically this change
could reduce compile times because of less header parsing.
[0]: 1bf6d8b0b9
Pull Request: https://projects.blender.org/blender/blender/pulls/138872