libstrophe: Add "platforms" meta information

pkgs/development/libraries/libstrophe/default.nix

···
     '';
     homepage = http://strophe.im/libstrophe/;
     license = stdenv.lib.licenses.gpl3;
+    platforms = stdenv.lib.platforms.linux;
     maintainers = [ stdenv.lib.maintainers.devhell ];
   };
 }