perf tools: Handle the case with and without the "signed" trace field

The trace format files now have a "signed" field. But we should
still be able to handle the kernels that do not have this field.

Signed-off-by: Steven Rostedt <srostedt@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
LKML-Reference: <20091014194358.888239553@goodmis.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>