sshuttle: add license

pkgs/tools/security/sshuttle/default.nix

@@ -45,6 +45,7 @@ python3Packages.buildPythonApplication rec {
       target network (though it does require Python 2 at both ends).
       Works with Linux and Mac OS and supports DNS tunneling.
     '';
+    license = licenses.gpl2;
     maintainers = with maintainers; [ domenkozar ];
     platforms = platforms.unix;
   };