Add missing sample pubspec to workspace

Follow up to https://dart-review.googlesource.com/c/sdk/+/397164

Change-Id: I6c9197bce858cbb7c5af50cf532b0ff1ae7d6311
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/415580
Reviewed-by: Alexander Thomas <athom@google.com>
Commit-Queue: Sigurd Meldgaard <sigurdm@google.com>

pubspec.yaml

@@ -76,6 +76,7 @@ workspace:
   - pkg/vm_service_protos
   - pkg/vm_snapshot_analysis
   - pkg/wasm_builder
+  - samples/ffi/http
   # dap and language_server_protocol are checked in to and
   # developed in the sdk repo, though they are located in `third_party/`.
   - third_party/pkg/dap