tracing: hide unused ftrace_event_id_fops

tracing: hide unused ftrace_event_id_fops

When CONFIG_PERF_EVENTS, a 'make W=1' build produces a warning about the
unused ftrace_event_id_fops variable:

kernel/trace/trace_events.c:2155:37: error: 'ftrace_event_id_fops' defined but not used [-Werror=unused-const-variable=]
 2155 | static const struct file_operations ftrace_event_id_fops = {

Hide this in the same #ifdef as the reference to it.

Link: https://lore.kernel.org/linux-trace-kernel/[email protected]
Cc: Masami Hiramatsu <[email protected]>
Cc: Oleg Nesterov <[email protected]>
Cc: Mathieu Desnoyers <[email protected]>
Cc: Zheng Yejian <[email protected]>
Cc: Kees Cook <[email protected]>
Cc: Ajay Kaher <[email protected]>
Cc: Jinjie Ruan <[email protected]>
Cc: Clément Léger <[email protected]>
Cc: Dan Carpenter <[email protected]>
Cc: "Tzvetomir Stoyanov (VMware)" <[email protected]>
Fixes: 620a30e97feb ("tracing: Don't pass file_operations array to event_create_dir()")
Signed-off-by: Arnd Bergmann <[email protected]>
Signed-off-by: Steven Rostedt (Google) <[email protected]>