watchlog: Remove Darwin from supported platforms.

pkgs/tools/misc/watchlog/default.nix

···
     homepage = "https://gitlab.com/kevincox/watchlog";
     license = lib.licenses.asl20;
     maintainers = with lib.maintainers; [ kevincox ];
+
+    # Dependency only supports Linux + Windows: https://github.com/mentaljam/standard_paths/tree/master/src
+    platforms = with lib.platforms; linux ++ windows;
   };
 }