Motivation for this change

Greatly decreases zbar-lib transitive closure size: assertion messages
contain paths to Qt header files, causing zbar-lib to depend on
qtbase-dev.

The Qt specific mkDerivation normally does this automatically, but we
can't use it here due to having only a small part of the project being
Qt related.

Before/after:

$ nix path-info -Sh /nix/store/yxcljqcpg03qrgp9cagvclkwx0rllq7m-zbar-0.23-lib
/nix/store/yxcljqcpg03qrgp9cagvclkwx0rllq7m-zbar-0.23-lib          1.1G
$ nix path-info -Sh /nix/store/864nvvs52wcplg8xslzhc54l5msiyngq-zbar-0.23-lib
/nix/store/864nvvs52wcplg8xslzhc54l5msiyngq-zbar-0.23-lib        493.0M

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nix-review --run "nix-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.