bson: provide required-but-missing virtual destructor for virtual base class

[nabijaczleweli]

In file included from src/bsontools/bsonmain.cpp:10:
In file included from /usr/lib/gcc/x86_64-linux-gnu/12/../../../../include/c++/12/memory:76:
/usr/lib/gcc/x86_64-linux-gnu/12/../../../../include/c++/12/bits/unique_ptr.h:95:2: warning: delete called on 'bsontools::dom::node' that is abstract but has non-virtual destructor [-Wdelete-abstract-non-virtual-dtor]
   95 |         delete __ptr;
      |         ^
/usr/lib/gcc/x86_64-linux-gnu/12/../../../../include/c++/12/bits/unique_ptr.h:396:4: note: in instantiation of member function 'std::default_delete<bsontools::dom::node>::operator()' requested here
  396 |           get_deleter()(std::move(__ptr));
      |           ^
src/bsontools/bsonmain.cpp:170:40: note: in instantiation of member function 'std::unique_ptr<bsontools::dom::node>::~unique_ptr' requested here
  170 |                     children.push_back(unique_ptr<node>(s));
      |                                        ^
1 warning generated.

This means that derived classes' destructors won't get called.