perf_event: Fix compile error

Fix:

 cc1: warnings being treated as errors
 builtin-probe.c: In function 'cmd_probe':
 builtin-probe.c:163: error: unused variable 'fd'

Signed-off-by: Xiao Guangrong <xiaoguangrong@cn.fujitsu.com>
Cc: Masami Hiramatsu <mhiramat@redhat.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
LKML-Reference: <4B162089.8000907@cn.fujitsu.com>
[ v2: use NO_LIBDWARF instead of __used ]
Signed-off-by: Ingo Molnar <mingo@elte.hu>

tools/perf/builtin-probe.c

@@ -160,7 +160,10 @@ static const struct option options[] = {
 
 int cmd_probe(int argc, const char **argv, const char *prefix __used)
 {
-	int i, j, fd, ret;
+	int i, j, ret;
+#ifndef NO_LIBDWARF
+	int fd;
+#endif
 	struct probe_point *pp;
 
 	argc = parse_options(argc, argv, options, probe_usage,