as `my_result_collector` has virtual function, and its dtor is not
marked virtual, Clang complains. let's mark its base class virtual
to be on the safe side.
```
/home/kefu/.local/bin/../lib/gcc/x86_64-pc-linux-gnu/13.0.1/../../../../include/c++/13.0.1/bits/unique_ptr.h:100:2: error: delete called on non-final 'my_result_collector' that has virtual functions but non-virtual destructor [-Werror,-Wdelete-non-abstract-non-virtual-dtor]
delete __ptr;
^
/home/kefu/.local/bin/../lib/gcc/x86_64-pc-linux-gnu/13.0.1/../../../../include/c++/13.0.1/bits/unique_ptr.h:405:4: note: in instantiation of member function 'std::default_delete<my_result_collector>::operator()' requested here
get_deleter()(std::move(__ptr));
^
/home/kefu/dev/scylladb/db/virtual_table.cc:134:25: note: in instantiation of member function 'std::unique_ptr<my_result_collector>::~unique_ptr' requested here
auto consumer = std::make_unique<my_result_collector>(s, permit, &pr, std::move(reader_and_handle.second));
^
```
Signed-off-by: Kefu Chai <kefu.chai@scylladb.com>
Closes#12879
2f0cb9e68f