The foreign_ptr wrapper needs 'element_type' to be present in
shared_ptr to be able to access the data.
Fixes the following compilation failure when trying to use shared_ptr
with foreign_ptr:
In file included from tests/foreign_ptr_test.cc:24:0:
./core/distributed.hh: In instantiation of ‘class foreign_ptr<shared_ptr<basic_sstring<char, unsigned int, 15u> > >’:
tests/foreign_ptr_test.cc:28:54: required from here
./core/distributed.hh:272:56: error: no type named ‘element_type’ in ‘class shared_ptr<basic_sstring<char, unsigned int, 15u> >’
using element_type = typename PtrType::element_type;
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
