Fix failing build on MacOS (#2313)
Fix failing build on MacOS (#2313)

# Motivation
Currently, the main branch fails to build for me since the `testSimpleMPTCP()` can't compile. This is due to the fact that the `getMPTCPInfo()` method is only available on Linux.

# Modification
Add `#if os(Linux)` around the `testSimpleMPTCP()`.

# Result
Main builds on MacOS again.
apple/swift-nio,apple/swift-nio,apple/swift-nio,apple/swift-nio,apple/swift-nio