home: ssh: use 'mkMerge'

This will make it easier to add new options (e.g: for `mosh`).

modules/home/ssh/default.nix

@@ -7,7 +7,9 @@ in
     enable = my.mkDisableOption "ssh configuration";
   };
 
-  config.programs.ssh = lib.mkIf cfg.enable {
+  config = lib.mkIf cfg.enable (lib.mkMerge [
+    {
+      programs.ssh = {
         enable = true;
 
         includes = [
@@ -52,3 +54,5 @@ in
         '';
       };
     }
+  ]);
+}