Add missing "flags" argument to wxNotificationMessage ctor in wxMSW.

Add missing "flags" argument to wxNotificationMessage ctor in wxMSW.

It was impossible to specify the flags when creating wxNotificationMessage in
wxMSW, cotrary to the other ports and the documentation.

Fix this by adding the missing argument to the ctor and simply passing it to
the base class one, we don't have to do anything else as the existing code
already calls GetFlags() later so it should respect these flags without any
more changes.

Closes #14245.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@71312 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775