gate: add default move assignment operator
gate: add default move assignment operator

d240dac462cfa7287c225e386a43a12a68b9d87f seems to have caused the
default move assignment operator to be deleted.

Signed-off-by: Noah Watkins <1db01d43e08596f43a65fb393d969b98ee5b4dc6@vectorized.io>
Message-Id: <20210514024515.1283534-1-1db01d43e08596f43a65fb393d969b98ee5b4dc6@vectorized.io>

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