Add missing platform specifications

Author: gwynne

Package.swift

@@ -4,6 +4,12 @@ import PackageDescription
 
 let package = Package(
     name: "swift-http-types",
+    platforms: [
+        .macOS("10.15"),
+        .iOS("13.0"),
+        .watchOS("6.0"),
+        .tvOS("13.0"),
+    ],
     products: [
         .library(name: "HTTPTypes", targets: ["HTTPTypes"]),
         .library(name: "HTTPTypesFoundation", targets: ["HTTPTypesFoundation"]),