isolate.gypi: Only load build/util/version.gypi for Windows.
isolate.gypi: Only load build/util/version.gypi for Windows.

It was added back in https://codereview.chromium.org/140263002 in
order to load chrome.exe on Windows.

BUG=603402

Review URL: https://codereview.chromium.org/1891763002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#387309}

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