dnsdist: fix finding boost

https://hydra.nixos.org/build/302826781/nixlog/2/tail
It's similar to a few other packages in this staging-next cycle.

pkgs/by-name/dn/dnsdist/package.nix

···
     "--disable-dependency-tracking"
     "--enable-unit-tests"
     "--enable-systemd"
+    "--with-boost=${boost.dev}"
   ];
 
   doCheck = true;