Added missing availability check for tvOS 13.0

Sources/Utility/String+SHA256.swift

@@ -32,7 +32,7 @@ extension String: KingfisherCompatibleValue { }
 extension KingfisherWrapper where Base == String {
     var sha256: String {
         guard let data = base.data(using: .utf8) else { return base }
-        if #available(iOS 13.0, *) {
+        if #available(iOS 13.0, tvOS 13.0, *) {
             let hashed = SHA256.hash(data: data)
             return hashed.compactMap { String(format: "%02x", $0) }.joined()
         } else {