cbc: Enable on Darwin

pkgs/applications/science/math/cbc/default.nix

···
     homepage = https://projects.coin-or.org/Cbc;
     license = lib.licenses.epl10;
     maintainers = [ lib.maintainers.eelco ];
-    platforms = lib.platforms.linux;
+    platforms = lib.platforms.linux ++ lib.platforms.darwin;
     description = "A mixed integer programming solver";
   };
 }