when compiling with Clang-18 + libstdc++-13, the tree fails to build:
```
/home/kefu/dev/scylladb/tasks/task_manager.hh:45:36: error: no template named 'list' in namespace 'std'
45 | using foreign_task_list = std::list<foreign_task_ptr>;
| ~~~~~^
```
so let's include the used header
Signed-off-by: Kefu Chai <kefu.chai@scylladb.com>
Closesscylladb/scylladb#16433