pkgs charybdis: change platforms to linux only

pkgs/servers/irc/charybdis/default.nix

···
     homepage    = https://github.com/atheme/charybdis;
     license     = stdenv.lib.licenses.gpl2;
     maintainers = [ stdenv.lib.maintainers.lassulus ];
-    platforms   = stdenv.lib.platforms.all;
+    platforms   = stdenv.lib.platforms.linux;
   };