Tests link with libutil for openpty on OpenBSD.

[3405691582]

One of the tests refers to openpty, which requires linking with libutil on OpenBSD.

Motivation:

swift test on the project has a link failure on OpenBSD.

Modifications:

Add the libutil linker flag in Package.swift

Checklist:

• Code and documentation should follow the style of the Style Guide.
• If public symbols are renamed or modified, DocC references should be updated. (not required).

[stmontgomery]

@swift-ci please test

[3405691582]

Please merge on my behalf if there's nothing else, thank you!

[stmontgomery]

Thanks @3405691582. For reference, I see the reference to openpty here:

swift-testing/Tests/TestingTests/Support/FileHandleTests.swift

Line 143 in 887871b

try #require(0 == openpty(&primary, &secondary, nil, nil, nil))