Disable creation of wxDIB from wxBitmap in non-wxMSW ports.

wxDIB can now be used with wxGTK under Windows, i.e. when not using wxMSW
version of wxBitmap but it can't be created from wxBitmap then as it doesn't
have a corresponding HBITMAP.

Simply disable this functionality when not using wxMSW.

Closes #14414.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@71798 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775