src.ctf.fs: make ctf_visitor_generate_ir::trace_class a bt2::TraceClass::Shared

src.ctf.fs: make ctf_visitor_generate_ir::trace_class a bt2::TraceClass::Shared

Change-Id: I9b414384b025f39c3cfb9900659235a17426b649
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/8283
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/12321
Tested-by: jenkins <jenkins@lttng.org>