schema_tables: adjust altered_schema construction for clang
schema_tables: adjust altered_schema construction for clang

Clang does not implement P0960R3, parenthesized initialization of
aggregates, so we have to use brace initialization in
altered_schema.

As the parenthesized constructor call is done by emplace_back(),
we have to do the braced call ourselves.

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