variant_utils: add variant_visitor constructor for C++17 mode
variant_utils: add variant_visitor constructor for C++17 mode

Fixes build in C++17 mode.

cloudius-systems/seastar,scylladb/seastar,syuu1228/seastar,avikivity/seastar,scylladb/seastar,cloudius-systems/seastar,dreamsxin/seastar,avikivity/seastar,cloudius-systems/seastar,dreamsxin/seastar,dreamsxin/seastar,syuu1228/seastar,syuu1228/seastar,avikivity/seastar,scylladb/seastar