Suppress warnings on Windows too

[dabrahams]

Bug/issue #, if applicable:

Summary

When I build on Windows, I get multiple copies of this warning:

C:\Users\dave\src\hylo\.build\checkouts\swift-docc-plugin\Plugins\Swift-DocC Convert\Symbolic Links\SharedPackagePluginExtensions\PackageExtensions.swift:31:17: warning: switch covers known cases, but 'TargetDependency' may have additional unknown values
                switch dependency {
                ^

Checklist

Make sure you check off the following items. If they cannot be completed, provide a reason.

• [-] Added tests Nothing new to test
• Ran the ./bin/test script and it succeeded
• Updated documentation if necessary

[d-ronnqvist]

@swift-ci please test

[ktoso]

Seems this also resolves #83