removed the executable flag for three rust source files

[alexanderkjall]

because this causes problems when packaging the crate as a debian source package

[lesniewski]

@danielrh FYI

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[alexanderkjall]

I'm not interested in giving away my copyright for this PR, since the project is licensed under the 'BSD 3-Clause "New" or "Revised" License' I'm happy to provide this patch under the same license. If that's not acceptable feel free to email me and we can work out a contract where you buy the PR from me, or just close the PR and no hard feelings.