cql3: untyped_result_set.hh: add missing include for column_specification
cql3: untyped_result_set.hh: add missing include for column_specification

Fails dev-headers build without it.

Message-Id: <8cb414246b800d4ecd026aaa0c727da40e48fb44@scylladb.com>

scylladb/scylla,avikivity/scylla,avikivity/scylla,avikivity/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla