Add missing Dispatch import (#223)

Motivation:

Dispatch wasn't imported in `NIOTSSingletons.swift` where `.default` was
used for the default QoS. This causes some build issues.

Modifications:

- Add missing Dispatch import

Result:

Fewer build issues

---------

Co-authored-by: Cory Benfield <lukasa@apple.com>

Sources/NIOTransportServices/NIOTSSingletons.swift

@@ -13,6 +13,7 @@
 //===----------------------------------------------------------------------===//
 
 #if canImport(Network)
+import Dispatch
 import NIOCore
 
 @available(OSX 10.14, iOS 12.0, tvOS 12.0, watchOS 6.0, *)

Sources/NIOTransportServices/StateManagedNWConnectionChannel.swift

@@ -13,6 +13,7 @@
 //===----------------------------------------------------------------------===//
 
 #if canImport(Network)
+import Atomics
 import Foundation
 import NIOCore
 import NIOConcurrencyHelpers

Tests/NIOTransportServicesTests/NIOTSConnectionChannelTests.swift

@@ -16,6 +16,7 @@
 import XCTest
 import Network
 import NIOCore
+import NIOFoundationCompat
 import NIOTransportServices
 import Foundation