fix compilation with cygwin

[kroening]

The variant of std::ifstream with a wide character file name is only
available when using Visual Studio but not when using Cygwin.

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• n/a Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• n/a The feature or user visible behaviour I have added or modified has been documented in the User Guide in doc/cprover-manual/
• Regression or unit tests are included, or existing tests cover the modified code (in this case I have detailed which ones those are in the commit message).
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• My PR is restricted to a single feature or bugfix.
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[allredj]

⚠️
This PR failed Diffblue compatibility checks (cbmc commit: 740fa25).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/106477586
Status will be re-evaluated on next push.
Common spurious failures include: the cbmc commit has disappeared in the mean time (e.g. in a force-push); the author is not in the list of contributors (e.g. first-time contributors); compatibility was already broken by an earlier merge.

[allredj]

✔️
Passed Diffblue compatibility checks (cbmc commit: f43e786).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/106478182