tools lib traceevent: Add __maybe_unused to unused parameters

Fixing the build on 32-bit Fedora 14:

  tools/lib/traceevent/event-parse.c: In function ‘print_event_fields’:
  tools/lib/traceevent/event-parse.c:3934:69: error: unused parameter ‘size’
  tools/lib/traceevent/event-parse.c: In function ‘pevent_strerror’:
  tools/lib/traceevent/event-parse.c:5074:36: error: unused parameter ‘pevent’

Cc: David Ahern <dsahern@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Link: http://lkml.kernel.org/n/tip-soe4gqcz8fd4ecik6exvyqox@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>