libguestfs: build on hydra

It used to be huge, but current measurements reveal a 48mb NAR

pkgs/development/libraries/libguestfs/default.nix

@@ -85,6 +85,5 @@ stdenv.mkDerivation rec {
     homepage = http://libguestfs.org/;
     maintainers = with maintainers; [offline];
     platforms = platforms.linux;
-    hydraPlatforms = [];
   };
 }