Added DSL error for opaque types in structs

Change-Id: I3fc35b2d5c5c0d1b533c165496d604fb7ac6e119
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/443178
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
Reviewed-by: John Stiles <johnstiles@google.com>
1 file changed