Add missing dependency on pkg:ffi

This does not enable the test, but it does silence warnings when publishing

PiperOrigin-RevId: 338124273
diff --git a/pubspec.yaml b/pubspec.yaml
index 5210876..9bbb27d 100644
--- a/pubspec.yaml
+++ b/pubspec.yaml
@@ -15,10 +15,15 @@
   path: ^1.8.0-nullsafety.1
 
 dev_dependencies:
+  ffi: any
   fixnum: '>=0.9.0 <0.11.0'
   js: 0.6.3-nullsafety
   test: ^1.16.0-nullsafety.4
 
 dependency_overrides:
+  ffi:
+    git:
+      url: https://github.com/dart-lang/ffi
+      ref: null_safety
   fixnum:
     git: https://github.com/dart-lang/fixnum