core,base-sim: fix warnings with -Wswitch-default

mm-base-sim.c: In function ‘interface_initialization_step’:
  mm-base-sim.c:1587:5: warning: switch missing default case [-Wswitch-default]
   1587 |     switch (ctx->step) {
        |     ^~~~~~

src/mm-base-sim.c

@@ -1674,6 +1674,9 @@ interface_initialization_step (GTask *task)
         g_task_return_boolean (task, TRUE);
         g_object_unref (task);
         return;
+
+    default:
+        break;
     }
 
     g_assert_not_reached ();