add viewer/windows files

example/windows/flutter/generated_plugin_registrant.h

@@ -0,0 +1,15 @@
+//
+//  Generated file. Do not edit.
+//
+
+// clang-format off
+
+#ifndef GENERATED_PLUGIN_REGISTRANT_
+#define GENERATED_PLUGIN_REGISTRANT_
+
+#include <flutter/plugin_registry.h>
+
+// Registers Flutter plugins.
+void RegisterPlugins(flutter::PluginRegistry* registry);
+
+#endif  // GENERATED_PLUGIN_REGISTRANT_