Fix carthage build failure

Author: tackgyu

Sources/Operators/PrefixDuration.swift

@@ -6,7 +6,7 @@
 //  Copyright © 2020 Combine Community. All rights reserved.
 //
 
-#if canImport(Combine)
+#if !(os(iOS) && (arch(i386) || arch(arm))) && canImport(Combine)
 import Combine
 import Foundation
 

Sources/Subjects/ReplaySubject.swift

@@ -8,6 +8,7 @@
 
 #if canImport(Combine)
 import Combine
+import Foundation
 
 /// A `ReplaySubject` is a subject that can buffer one or more values. It stores value events, up to its `bufferSize` in a
 /// first-in-first-out manner and then replays it to