Missing helpcontent2 png files

[Stephan Bergmann]

There are multiple recent help repo commits like 
<https://git.libreoffice.org/help/+/98f3d703be6e588c27a94e6ae41d5e4ce74cebbe%5E%21/> 
"Remove xmlsec screenshots (not used)" that remove lots of 
helpcontent2/source/media/screenshots/ png files.  But at least my local 
--with-help=html build now fails with

> /usr/bin/cp: cannot stat '/home/sberg/lo/core/helpcontent2/source/media/screenshots/modules/smath/ui/alignmentdialog/AlignmentDialog.png': No such file or directory
> make[1]: *** [/home/sberg/lo/core/solenv/gbuild/Package.mk:35: /home/sberg/lo/core/instdir/help/media/screenshots/modules/smath/ui/alignmentdialog/AlignmentDialog.png] Error 1

etc.