build: Fix compilation for C++17
build: Fix compilation for C++17

The implementation of `operator<<` for `optimized_optional` requires
that the `iostream` header is included.

Without this change, configuring Seastar with

    - --c++-dialect=gnu++17
    - --use-std-optional-variant-stringview=1

fails.

Fixes #580

Signed-off-by: Jesse Haber-Kucharsky <526eadea4792054b99d5fa5dd6be9acdd76c5418@scylladb.com>
Message-Id: <33ac7aac76c78fd0ee86f81ea35a4bbee63e5e5d.1547577904.git.526eadea4792054b99d5fa5dd6be9acdd76c5418@scylladb.com>

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