future-utils: avoid defining a template with its default template parameter
future-utils: avoid defining a template with its default template parameter

Clang doesn't like it.

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