perf trace: Hide sys_exit messages about syscall id = -1

perf trace: Hide sys_exit messages about syscall id = -1

That was reproduced via ftrace as described in this cset comment log,
need to investigate further.

Cc: Adrian Hunter <adrian.hunter@intel.com>
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>
Link: http://lkml.kernel.org/n/tip-n1i3m0vo6mgq3ddjj95sls2s@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>