remove usingnamespace from libxev and zig-protobuf
relevant PRs:
* libxev:
7f803181b1
* zig-protobuf: https://github.com/zml/zig-protobuf/pull/1/files
This means we can now build ZML with `-fincremental` (in debug mode only
and linux x86-64)
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Signed-off-by: Guillaume Wenzek <wzk@zml.ai>
