Mark ffi vfs test as using ffi

Author: simolus3

sqlite3/test/ffi/vfs_test.dart

@@ -1,4 +1,8 @@
+@Tags(['ffi'])
+library;
+
 import 'package:sqlite3/sqlite3.dart';
+import 'package:test/test.dart';
 
 import '../common/vfs.dart';