Speed up FileToFlatbuffer

This takes one test down from 1500ms to 500ms running under
fastbuild-msan.

Change-Id: I9bd3e7b1d566b7c9362319eaa21d9af86395f41e
diff --git a/aos/BUILD b/aos/BUILD
index 0159623..04293df 100644
--- a/aos/BUILD
+++ b/aos/BUILD
@@ -335,6 +335,7 @@
         ":flatbuffer_utils",
         ":flatbuffers",
         ":json_tokenizer",
+        "//aos/util:file",
         "@com_github_google_flatbuffers//:flatbuffers",
         "@com_github_google_glog//:glog",
         "@com_google_absl//absl/strings",