cql3: Make column_specification non-final

It's inherited by the config.ColumnDefinition class.

Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>

cql3/column_specification.hh

@@ -31,7 +31,7 @@
 
 namespace cql3 {
 
-class column_specification final {
+class column_specification {
 public:
     const sstring ks_name;
     const sstring cf_name;