perf tools: Add 'signed' flag setting back into trace-event-parse.c

Commit 13999e59 (perf tools:
Handle the case with and without the "signed" trace field)
removed code to set the FIELD_IS_SIGNED flag that was originally
added by commit 26a50744
(tracing/events: Add 'signed' field to format files).

This adds it back.

Signed-off-by: Tom Zanussi <tzanussi@gmail.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
LKML-Reference: <1259133299-23594-2-git-send-email-tzanussi@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>