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>