WinSDK: extract WinRPC into a module

Add a module representing RpcRT4.  This is needed for interfacing with
certain COM interfaces.

Author: compnerd

stdlib/public/Platform/winsdk.modulemap

@@ -216,5 +216,13 @@ module WinSDK [system] {
 
     link "AdvAPI32.Lib"
   }
+
+  module WinRPC {
+    header "../shared/rpc.h"
+    header "../shared/rpcndr.h"
+    export *
+
+    link "RpcRT4.Lib"
+  }
 }