[PR #1603] Added Foundation import to DispatchQueue extension.

Without this, Alamofire was failing to compile when used as a submodule.

Source/DispatchQueue+Alamofire.swift

@@ -23,6 +23,7 @@
 //
 
 import Dispatch
+import Foundation
 
 extension DispatchQueue {
     static var userInteractive: DispatchQueue { return DispatchQueue.global(qos: .userInteractive) }