ldns: relax platform requirements

Seems to have built OK on darwin. I only needed it to build freeswitch
though, so not sure how to test functionality. I successfully ran the
`drill` command.

pkgs/development/libraries/ldns/default.nix

···
     description = "Library with the aim of simplifying DNS programming in C";
     license = licenses.bsd3;
     homepage = http://www.nlnetlabs.nl/projects/ldns/;
-    platforms = platforms.linux;
+    platforms = platforms.unix;
     maintainers = with maintainers; [ jgeerds ];
   };
 }