this change was created in the same spirit of aebb5329, which
included the fmt/iostream.h and iostream when appropriate so that
the tree can build with seastar submodule including e96932b0.
in the seastar change, we stopped including unused `fmt/ostream.h`
in a public header in seastar, so the parent projects relying on
the header to indirectly include fmt/ostream.h and iostream would
have to include these headers explicitly.
Signed-off-by: Kefu Chai <kefu.chai@scylladb.com>
Closesscylladb/scylladb#21525