Add package:ffi as path dependency to flutter_frontend_server (flutter#37802)

The CL in [0] added package:ffi as dependency to package:compiler. Since
flutter_frontend_server needs all dependencies as path dependencies, we
need to add package:ffi there via path.

[0] https://dart-review.googlesource.com/c/sdk/+/269640

Author: mkustermann

flutter_frontend_server/pubspec.yaml

@@ -55,6 +55,8 @@ dependency_overrides:
     path: ../../third_party/dart/pkg/dev_compiler
   expect:
     path: ../../third_party/dart/pkg/expect
+  ffi:
+    path: ../../third_party/dart/third_party/pkg/ffi
   fixnum:
     path: ../../third_party/dart/third_party/pkg/fixnum
   front_end: