rpc: make unmarshall_exception() inline
rpc: make unmarshall_exception() inline

gcc 7 complains that unmarshall_exception is not used. Mark it inline
so it doesn't.
Message-Id: <35cea383f45a4e1a8ea4d1470a487b82c34bdccc@scylladb.com>

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