there is chance that `utils/small_vector.hh` does not include
`using namespace seastar`, and even if it does, we should not rely
on it. but if it does not, checkhh would fail. so let's include
"seastarx.hh" in this header, so it is self-contained.
Signed-off-by: Kefu Chai <kefu.chai@scylladb.com>
Closesscylladb/scylladb#18265