Add missing platform specifiers (#146)

Author: gwynne

Package.swift

@@ -4,7 +4,10 @@ import PackageDescription
 let package = Package(
     name: "jwt",
     platforms: [
-       .macOS(.v10_15)
+        .macOS(.v10_15),
+        .iOS(.v13),
+        .tvOS(.v13),
+        .watchOS(.v6)
     ],
     products: [
         .library(name: "JWT", targets: ["JWT"]),