[protobuf] add support for mingw subsystem

[fairlyblank]

Describe the pull request

• What does your PR fix?

  protobuf build for mingw subsystem. no issue.

• Which triplets are supported/not supported? Have you updated the CI baseline?

  all, No

• Does your PR follow the maintainer guide?

  Yes

• If you have added/updated a port: Have you run ./vcpkg x-add-version --all and committed the result?

  Yes

If you are still working on the PR, open it as a Draft: https://github.blog/2019-02-14-introducing-draft-pull-requests/

[PhoebeHui]

@fairlyblank, could you update the baseline version via './vcpkg x-add-version --overwrite-version protobuf' and submit the changes again?

[fairlyblank]

@fairlyblank, could you update the baseline version via './vcpkg x-add-version --overwrite-version protobuf' and submit the changes again?

I think i have updated the git-tree in last commit, after 'git rev-parse HEAD:ports/protubuf' .
I have run './vcpkg x-add-version --overwrite-version protobuf', but nothing need to commit.
Something i'm doing wrong here?

[PhoebeHui]

@fairlyblank, you didn't update the port-version in vcpkg.json, so it failed, I have updated it and committed the changes.

[fairlyblank]

@fairlyblank, you didn't update the port-version in vcpkg.json, so it failed, I have updated it and committed the changes.

I missed that, thanks for your time.