perf tools: Continue processing header on unknown features

A feature may be unknown if perf.data is created and parsed on different
perf tool versions. This should not stop the header to be processed,
instead continue processing it.

Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/r/1323248577-11268-2-git-send-email-robert.richter@amd.com


Signed-off-by: Robert Richter <robert.richter@amd.com>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>