Add plugin_registrar_windows.h to published wrapper (flutter#15265)

Author: stuartmorgan-g

shell/platform/windows/client_wrapper/BUILD.gn

@@ -5,7 +5,10 @@
 import("$flutter_root/shell/platform/common/cpp/client_wrapper/publish.gni")
 import("$flutter_root/testing/testing.gni")
 
-_wrapper_includes = [ "include/flutter/flutter_view_controller.h" ]
+_wrapper_includes = [
+  "include/flutter/flutter_view_controller.h",
+  "include/flutter/plugin_registrar_windows.h",
+]
 
 _wrapper_sources = [ "flutter_view_controller.cc" ]