fix: sealing: stub out the FileSize function on Windows

[aschmahmann]

Proposed Changes

Mocking out the FileSize function call in the sealer when built on Windows so that lotus library components can be used on Windows.

Additional Info

Would also be great to test that Windows library usage (e.g. of the state tree and actors) can continue to work on Windows even though running lotus and doing things like sealing isn't going to happen.

Checklist

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