ibus-packages.mozc: pin to protobuf 3.6 (fixes build breakage at HEAD)

[delroth]

Motivation for this change

google/mozc#460

mozc currently doesn't build on trunk: https://hydra.nixos.org/build/92239027

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nix-review --run "nix-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Assured whether relevant documentation is up to date
• Fits CONTRIBUTING.md.

---

[grahamc]

cc @7c6f434c did I see you making changes relevant to this?

[7c6f434c]

I fixed a similar but different problem: protobufc has unreleased upstream fix for a different protobuf API change.

[grahamc]

@GrahamcOfBorg build ibus-packages.mozc

[grahamc]

turns out it is called:

@GrahamcOfBorg build ibus-engines.mozc

[delroth]

Thank you! 👍

[Arfrever]

Motivation for this change

google/mozc#460

This would be relevant with Protobuf 3.7.0, but not with Protobuf 3.7.1.

https://github.com/protocolbuffers/protobuf/releases/tag/v3.7.1 says:

Avoid marking generated C++ messages as final, though we will do this in a future release

I do not use NixOS, but in Gentoo I have successfully built Mozc with Protobuf 3.7.1.
In https://github.com/NixOS/nixpkgs/blob/985b9e47b8f4e96fcf4dc691e69e7813827b7ee5/pkgs/development/libraries/protobuf/3.7.nix I see:

 version = "3.7.0";

If you update it to 3.7.1, then it should also work...