reaver-wps: set platforms

pkgs/tools/networking/reaver-wps/default.nix

···
     description = "Brute force attack against Wifi Protected Setup";
     homepage = http://code.google.com/p/reaver-wps;
     license = stdenv.lib.licenses.gpl2Plus;
+    platforms = stdenv.lib.platforms.linux;
   };
 }