[fuchsia] RTTI fix.

Change-Id: I7669cf08708551047d601cdff0b6e0a2e4fe149e
Reviewed-on: https://fuchsia-review.googlesource.com/c/third_party/mesa/+/1050931
Reviewed-by: Craig Stout <cstout@google.com>
Commit-Queue: Auto-Submit <auto-submit@fuchsia-infra.iam.gserviceaccount.com>
Fuchsia-Auto-Submit: Josh Gargus <jjosh@google.com>