Crash build on Linux

[bordunosp]

There is a problem when trying to build a project on Linux

.build/checkouts/Async/Sources/Async/Async.swift:735:18: error: cannot find type 'qos_class_t' in scope
public extension qos_class_t {
                 ^~~~~~~~~~~
.build/checkouts/Async/Sources/Async/Async.swift:767:45: error: cannot find 'qos_class_main' in scope
        case DispatchQoS.QoSClass(rawValue: qos_class_main())!: result = .main

[natinusala]

FWIW there is the exact same error on Windows.