tools lib traceevent: Print value of unknown symbolic fields

Aligns trace-cmd with the behavior of the kernel.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Acked-by: Namhyung Kim <namhyung@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Link: http://lkml.kernel.org/r/e60c889f-55e7-4ee8-0e50-151e435ffd8c@siemens.com
Link: http://lkml.kernel.org/r/20180112004822.118332436@goodmis.org


Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>