kappanhang: restrict platforms to linux-only

According to the README:

kappanhang currently only supports Linux, but support for other platforms
can be easily added if anyone is interested and has the skills
(volunteers needed, as I'm only developing the Linux version).

pkgs/applications/radio/kappanhang/default.nix

···
     homepage = "https://github.com/nonoo/kappanhang";
     description = "Remote control for Icom radio transceivers";
     license = licenses.mit;
+    platforms = platforms.linux;
     maintainers = with maintainers; [ mvs ];
   };
 }