Switch to c++11 enums for flatbuffers

Change-Id: I99171464bf54efbb7eb322c556760dee86648f5e
diff --git a/third_party/flatbuffers/build_defs.bzl b/third_party/flatbuffers/build_defs.bzl
index facb24b..da61852 100644
--- a/third_party/flatbuffers/build_defs.bzl
+++ b/third_party/flatbuffers/build_defs.bzl
@@ -22,6 +22,7 @@
     "--reflect-names",
     "--cpp-ptr-type flatbuffers::unique_ptr",
     "--force-empty",
+    "--scoped-enums",
     "--gen-name-strings",
 ]