tools lib traceevent: Avoid comparisions between signed/unsigned

Fixing this warning-as-error on f14 32-bit:

  tools/lib/traceevent/event-parse.c:5564:17: error: comparison between signed and unsigned integer expressions
  tools/lib/traceevent/event-parse.c:5586:17: error: comparison between signed and unsigned integer expressions

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-stmix8hy4nu5ervpynn8yj2z@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>