Add libispc1 and libispc-dev packages for new library

@mzf I did a team upload now to update ispc to the latest vesion so that it builds with llvm 21. Debian hasn't yet started this transition (llvm 20 is being skipped), but Ubuntu has.

The new version adds a new library (see the 1.28.0 release notes) so that will need to go through the NEW queue to enable it. I simply disabled the library for my upload since it's not needed yet and to let you have the opportunity to review before an upload to NEW.

I didn't have the new -dev package depend on ispc. Maybe the existing -dev package shouldn't either?

I did not add an autopkgtest for the new library.