libguestfs: Don't build on Hydra

It produces a package over 4 GB large, and takes several hours to
build (probably because of the overhead of copying that 4 GB image).

pkgs/development/libraries/libguestfs/default.nix

@@ -51,5 +51,6 @@ stdenv.mkDerivation rec {
     homepage = http://libguestfs.org/;
     maintainers = with maintainers; [offline];
     platforms = with platforms; linux;
+    hydraPlatforms = [];
   };
 }