Require explicit IDs for flatbuffers

This will hopefully make it a lot harder for us to accidentally change
the binary schema.

Change-Id: Ied685f7b6872031c0ebf7426aeadd203e5f1eace
diff --git a/third_party/flatbuffers/build_defs.bzl b/third_party/flatbuffers/build_defs.bzl
index f9c06f8..f496dab 100644
--- a/third_party/flatbuffers/build_defs.bzl
+++ b/third_party/flatbuffers/build_defs.bzl
@@ -22,6 +22,7 @@
     "--keep-prefix",
     "--cpp-std",
     "c++17",
+    "--require-explicit-ids",
     "--gen-mutable",
     "--reflect-names",
     "--cpp-ptr-type flatbuffers::unique_ptr",