Support watchos in spm

The podspec supports watchos, but the spm metadata doesn't.

Package.swift

@@ -5,7 +5,7 @@ import PackageDescription
 let package = Package(
   name: "CryptoSwift",
   platforms: [
-    .macOS(.v10_12), .iOS(.v9), .tvOS(.v9)
+    .macOS(.v10_12), .iOS(.v9), .tvOS(.v9), .watchOS(.v2)
   ],
   products: [
     .library(