core/reactor: Add class keyword to friend declaration
core/reactor: Add class keyword to friend declaration

CLion gets confused without this.

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