Reverts a change made from our 'sync protos' script that shouldn't have been made.

Author: arcra

tensorboard/compat/proto/BUILD

@@ -238,6 +238,7 @@ tb_proto_library(
 tb_proto_library(
     name = "summary",
     srcs = ["summary.proto"],
+    exports = [":histogram"],
     deps = [
         ":histogram",
         ":tensor",