crashplan: specify meta.platforms to improve eval

If you tried to evaluate nixpkgs with allowUnfree and a non-glibc
platform, you would get an error when accessing stdenv.glibc.

pkgs/applications/backup/crashplan/crashplan-small-business.nix

@@ -96,6 +96,7 @@ stdenv.mkDerivation rec {
     description = "An online backup solution";
     homepage = http://www.crashplan.com/business/;
     license = licenses.unfree;
+    platforms = [ "x86_64-linux" ];
     maintainers = with maintainers; [ xvapx ];
   };