perf: Fix perf probe build error

When we run into dry run mode, we want to make
write_kprobe_trace_event to succeed on writing the event. Let's
initialize it to 0.

Fixes the following build error:
	util/probe-event.c:1266: attention : «ret» may be used uninitialized in this function
	util/probe-event.c:1266: note: «ret» was declared here

Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Acked-by: Masami Hiramatsu <mhiramat@redhat.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
LKML-Reference: <1271808065-25290-1-git-send-regression-fweisbec@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>