policykit: use meta.platforms

This makes it easier to remove policykit in the future

svn path=/nixpkgs/trunk/; revision=29594

pkgs/development/libraries/policykit/default.nix

···
   meta = {
     homepage = http://www.freedesktop.org/wiki/Software/PolicyKit;
     description = "A toolkit for defining and handling the policy that allows unprivileged processes to speak to privileged processes (obsolete)";
+    platforms = stdenv.lib.platforms.linux;
   };
 }

pkgs/top-level/release.nix

···
   pkgconfig = all;
   pltScheme = linux;
   pmccabe = linux;
-  policykit = linux;
   portmap = linux;
   postgresql = all;
   postfix = linux;