[Build] Add Protobuf flag to allow MSVC + Bazel

[tryuan99]

As mentioned in #49, this additional flag allows Protobuf to be successfully compiled on Windows.

See https://github.com/PisterLab/micromissiles-unity/actions/runs/16305928163 for a successful Windows build of the plugins.

[daniellovell]

Tested on my Windows machine with

bazel build -c opt //:plugins

Result:

Target //:plugins up-to-date:
  bazel-plugins/bazel-out/x64_windows-opt/bin/plugins.tar.gz
INFO: Elapsed time: 169.599s, Critical Path: 60.76s
INFO: 2118 processes: 835 internal, 1283 local.
INFO: Build completed successfully, 2118 total actions

Ran tests with

bazel test //assignment:all

Passed with result:

INFO: Elapsed time: 66.773s, Critical Path: 19.78s
INFO: 1158 processes: 991 action cache hit, 440 internal, 718 local.
INFO: Build completed successfully, 1158 total actions
//assignment:cover_assignment_test                                       PASSED in 0.2s
//assignment:even_assignment_test                                        PASSED in 0.3s
//assignment:weighted_even_assignment_test                               PASSED in 0.3s

Executed 3 out of 3 tests: 3 tests pass.
There were tests whose specified size is too big. Use the --test_verbose_timeout_warnings command line option to see which ones these are.

LGTM 👍 ship it