iostream: Constify eof() function
iostream: Constify eof() function

Signed-off-by: Duarte Nunes <a60e8b7496f7715d71354fe4c06dd24017025bf1@vectorized.io>
Message-Id: <20190905215915.12501-1-a60e8b7496f7715d71354fe4c06dd24017025bf1@vectorized.io>

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