build: export histogram proto declaration in summary proto (#6020)

This is needed to fix internal build after the `HistogramProto` location change in #6010. In OSS we don't need the `tb_proto_library` to actually use the `exports` arg since we could use the `public` symbol (https://developers.google.com/protocol-buffers/docs/proto3#importing_definitions) without additional changes. The signature change for `tb_proto_library` is required internally.

Corresponding internal change: cl/485927103 (see cl description for full details).

Author: yatbear

tensorboard/compat/proto/BUILD

@@ -237,6 +237,7 @@ tb_proto_library(
 tb_proto_library(
     name = "summary",
     srcs = ["summary.proto"],
+    exports = [":histogram"],
     deps = [
         ":histogram",
         ":tensor",

tensorboard/defs/protos.bzl

@@ -20,7 +20,9 @@ def tb_proto_library(
         deps = [],
         visibility = None,
         testonly = None,
-        has_services = False):
+        has_services = False,
+        # The `exports` arg is unused here, but required internally for compatibility.
+        exports = []):
     outs_proto = _PyOuts(srcs, grpc = False)
     outs_grpc = _PyOuts(srcs, grpc = True) if has_services else []
     outs_all = outs_proto + outs_grpc