oscar: Add include for untracked ssh_config

For example, this is useful for setting up temporary hosts that I don't
want tracked in my config. Sometimes I do this for e.g. vscodium remote
SSH.

oscar/configuration.nix

@@ -234,7 +234,9 @@
           user = "root";
         };
       };
-      extraConfig = "";
+      includes = [
+        "config.d/*"
+      ];
     };
 
     programs.firefox = {