oneko: enable darwin build

(cherry picked from commit ac02a1348f58cd5f1edb8fcff6e117c7ee160a73)

pkgs/applications/misc/oneko/default.nix

···
     homepage = "http://www.daidouji.com/oneko/";
     license = licenses.publicDomain;
     maintainers = [ maintainers.xaverdh ];
-    platforms = platforms.linux;
+    platforms = platforms.unix;
   };
 }