[vm/ffi] dart_api_dl.c fix MacOS build
[vm/ffi] dart_api_dl.c fix MacOS build

Fixes "redefinition of typedef '...' is a C11 feature" on Mac bots.

Change-Id: I9f1114279106fa83ca491e069564a9931541d6ad
Cq-Include-Trybots:dart/try:vm-kernel-mac-release-x64-try
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/158001
Auto-Submit: Daco Harkes <930cbae0ca77f1954c53e32620d89cfcca318f00@google.com>
Commit-Queue: Alexander Aprelev <dfb74c35a357c2059971a4bf7fe26b6a7c18085d@google.com>
Reviewed-by: Alexander Aprelev <dfb74c35a357c2059971a4bf7fe26b6a7c18085d@google.com>

dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk