because tracing/trace_keyspace_helper.cc references symbols
defined by table_helper, which is in turn provided by scylla-main,
we should link tracing_tracing against scylla-main.
otherwise we could have following link failure:
```
./build/./tracing/trace_keyspace_helper.cc:214: error: undefined reference to 'table_helper::setup_keyspace(cql3::query_processor&, service::migration_manager&, std::basic_string_view<char, std::char_traits<char> >, seastar::basic_sstring<char, unsigned int, 15u, true>, service::query_state&, std::vector<table_helper*, std::allocator<table_helper*> >)'
./build/./tracing/trace_keyspace_helper.cc:396: error: undefined reference to 'table_helper::cache_table_info(cql3::query_processor&, service::migration_manager&, service::query_state&)'
./table_helper.hh:92: error: undefined reference to 'table_helper::insert(cql3::query_processor&, service::migration_manager&, service::query_state&, seastar::noncopyable_function<cql3::query_options ()>)'
./table_helper.hh:92: error: undefined reference to 'table_helper::insert(cql3::query_processor&, service::migration_manager&, service::query_state&, seastar::noncopyable_function<cql3::query_options ()>)'
./table_helper.hh:92: error: undefined reference to 'table_helper::insert(cql3::query_processor&, service::migration_manager&, service::query_state&, seastar::noncopyable_function<cql3::query_options ()>)'
./table_helper.hh:92: error: undefined reference to 'table_helper::insert(cql3::query_processor&, service::migration_manager&, service::query_state&, seastar::noncopyable_function<cql3::query_options ()>)'
clang++-18: error: linker command failed with exit code 1 (use -v to see invocation)
ninja: build stopped: subcommand failed.
```
Signed-off-by: Kefu Chai <kefu.chai@scylladb.com>
Closesscylladb/scylladb#18455
94ac0799d9