tracing: Make trace_define_field_ext() static

trace_define_field_ext() is not used outside of trace_events.c, it should
be static.

Link: https://lore.kernel.org/oe-kbuild-all/202302130750.679RaRog-lkp@intel.com/

Fixes: b6c7abd1

 ("tracing: Fix TASK_COMM_LEN in trace event format file")
Reported-by: Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>