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.

Changed files
+1
pkgs
by-name
dn
dnsdist
+1
pkgs/by-name/dn/dnsdist/package.nix
···
"--disable-dependency-tracking"
"--enable-unit-tests"
"--enable-systemd"
+
"--with-boost=${boost.dev}"
];
doCheck = true;