Exclude doc package from flatpak build
The problem is that the BuildRequires ghostscript.spec in turn has a
Requires: %{_bindir}/dvips
which would expand to non-existent /app/bin/dvips when libtommath is built as a
flatpak (as a dependency of building libreoffice as a flatpak), which would
break the build. The easiest fix appears to be to just not build docs for the
flatpak case.