fix import of google/protobuf/timestamp.proto

[gecko655]

ref: bazel-contrib#2927

WORKSPACE file snippet:

git_repository(
    name = "io_bazel_rules_go",
    commit = "a6678449764fcd172550fe3218fa6aa03cc8bede",
    remote = "git@github.com:xflagstudio/rules_go.git",
    shallow_since = "1630391768 +0900",
)